The world has been in a dark state for 800 years. The entire story is happening because Joy Boy left a treasure, which Roger found. Roger arrived 'too early'. (The translation was 'too hasty', but I think it's now clear that the intent was literally that the Roger Pirates arrived too soon. They never did anything about what they learned. Roger disbanded his crew immediately afterward. They decided that the right decision was to wait. Shirahoshi wasn't born yet. The pieces were not yet in place. Roger was a dying man who did not have the time to do whatever needed to be done. He told the truth about the world to Whitebeard, he had a child with Rouge, and then he turned himself in. As Rayleigh said, he turned the flickering candle of his life into a roaring fire. Roger crafted his final words to initiate the Great Pirate era, to inspire a successor go find One Piece, as he did, and finish the things he could not. The one who discovers One Piece is the one who inherits Roger's will, as it has been handed down through the generations. The implication then in Rayleigh saying that the Straw Hats may arrive at a different conclusion is in reference to the inaction of the Roger Pirates. As stated, Roger decided to pass the message on and allow a successor to take up his mantle. Perhaps that was not the right decision. That is what the Straw Hats will have to decide.
These bigger things going on in the world are the main story. There is already a threat that Roger couldn't face. The implication of Teach becoming the main villain would be that Roger somehow fucked up royally by setting these events in motion because he created an even bigger threat than the one which has controlled the world for 800 years and is very the reason One Piece exists. This story is happening because of the greater enemy that Roger never faced.
Luffy and Teach are equally inspired by Roger's last words. Teach wouldn't even be a threat were it not for Roger's words - the very words that were meant to inspire a successor to face the enemy he could not. Roger set the events of the story into motion in order to achieve a goal. That goal is realized when the right person discovers his treasure and inherits his will. If it were possible without One Piece, there would be no point to the story. If it were possible for the Revolutionary Army or the 'good' Marines to fix things without the True History the efforts of the Kozuki Clan, the Fishmen, the Shandorans, the scholars of Ohara, it would have all been pointless. If Roger creates a bigger monster than the one he was trying to stop, Roger's plan would have been one huge mistake.
Blackbeard is a part of the story, yes. An important part? Definitely. He is the primary rival to inherit Roger's will. He is the one poised to threaten Luffy's claim to the title of Pirate King. But as Whitebeard said, it is not Teach. Whitebeard knew the truth of the wold. Luffy won't have to 'go start a war with the Government.' Whitebeard knew what the treasure was and said plainly that once it is discovered the world will be turned upside down. Doflamingo made the same remark about the secret of Marijoa. These are things that cannot be stopped. It isn't about Luffy going and starting a fight - but finishing something that is already in motion. Teach is a D. and though he is not the one who will discover One Piece, he is an important part of the story.
Rocks was not Roger's greatest enemy. Rocks was defeated and killed. His ambition was stopped. It may have been passed down to Teach. But Roger went on to have many more adventures after defeating Rocks. He learned the true history and discovered One Piece after the God Valley incident. Rocks was not the enemy that Roger failed to stop. Roger was not thinking of Rocks or Teach when he set the pirate age in motion.
Whitebeard's dying message was not about Teach. It was about the world and the war that will inevitably come when the treasure is found because the person who discovers it must pick up where Roger left off. Whitebeard knew what One Piece was. He had deep respect for Roger. He was raising Ace as his own son, with the intention that Ace would be the one to inherit his father's will and discover One Piece. Roger himself hoped that it would be his own child. But Ace died - and just as Luffy inherited the will of Roger, he came to also inherit the will of Ace. In his final moments Whitebeard looked at Teach with pity, not grave concern. He knows about the Will of D., One Piece, the true history - and he simply looked at Teach and said 'you are not the one Roger is waiting for.' Then he turned his attention to Sengoku and the rest of the world. Consider also that Whitebeard staked his claim to Fishman Island as his territory. Yes, this was partially done out of benevolence. But he also would have know the importance of the Mermaid Princess to the future. Whitebeard was protecting both Ace and Shirahoshi for the future.
I have tried to stick to the 'knowns' in my argument. The fact is, the things going on in the world represent a larger conflict - one which has been going on for much longer than this struggle for supremacy in the New WOrld. One Piece is seemingly the key to understanding that conflict and can only be discovered by the right person. Roger discovered this treasure, but was not able to do anything about it. He used his execution to spur on the whole world to search for One Piece. The very reason the story is happening is because of Roger's dying words and at that moment, he wasn't worried about Teach or Xebec. He was worried about the entity he was too early to face.
Teach is a 'dark' Luffy, yes. You could say he's Luffy's shadow. He is a representation of Luffy's 'shadow' (fittingly in possession of the shadow logia fruit). You could say that Roger's dying message along with Luffy meeting Shanks serve as Luffy's call to adventure. He leaves his ordinary world and embarks on an extraordinary quest. He has faced many challenges and made many allies. All the while he has been driven by his desire to achieve an impossible dream. Becoming Pirate King is not the true motivation for Luffy's adventure. Luffy doesn't want to 'conquer' anything; his motivating desire is freedom. In chapter 100, this quote from Roger accompanies Dragon's introduction: "Destiny. Dreams. An inherited strength of will. The ebb and flow of the ages. As long as people seek freedom in this life, these things shall not vanish from the Earth." Teach is the dark side of Luffy's ambition. He isn't an agent of chaos. He is an evil version of Luffy. In the hero's journey, at the innermost cave, the hero confronts their own shadow. Star Wars is a prime example as it drew heavily from Joseph Campbell's model of the cyclical hero's journey (or as he called it, the 'monomyth'); In the dark side cave on Dagobah, Luke faces an apparition of Vader and discovers his own face behind the mask. Luke later discovers that Vader is his father. Beyond the emotional turmoil presented by this unexpected familial connection, Luke has to confront the fact that the darkness he is trying to defeat is not an external force which can be eradicated as it resides within himself, within his own lineage, his own blood. This is what Teach represents in Luffy's journey. He is the dark side of Luffy's goal; the shadow. Heroes must face their shadow before they can go out and challenge the world. Campbell refers to this as 'returning with the elixir'; Carl Jung refers to it as 'synthesizing the shadow'. Star Wars represented this in Return of the Jedi - when Luke faces Vader again, he sees himself becoming the very thing he is trying to destroy as he is about to strike his father down in hatred and anger. He casts his lightsaber aside, recognizing and rejecting the dark side in himself. It cannot be destroyed, only recognized and kept in check. It is only then that Luke is able to confront the Emperor. Luke cannot physically challenge Sidious, but his resilience to torture is only possible because his own shadow has been synthesized. It is through this suffering that Vader is turned to the light and the Galaxy is ultimately saved. I am trying to make the point that in classic hero journeys, the confrontation with the 'dark self' or the 'shadow' is not the final act. It is that which precedes the moment of discover at the 'innermost cave' or the place in the unknown world where secret knowledge resides. The last leg of the journey is then taking that which the hero has learned and bringing it back to the ordinary world. Once the hero has confronted and synthesized their shadow, that is when they are ready to face the world and use that which they have learned to transform the society around them. It is a departure from the known, ordinary world into the special, unknown world. Descent. At the bottom, the hero faces the dark version of themself, learns something transformative, and then takes that knowledge or power back with them to the ordinary world.
I used Star Wars as a reference because perhaps more than any other story consciously draws on the archetypal hero's journey. As outlined in the Hero with a Thousand Faces, this is a pattern present in stories from around the world throughout time. It is not unique to western myths and folklore.
Teach is perfectly suited to challenge Luffy's right to inherit Roger's will. But the story is bigger than the search for One Piece. Roger found One Piece. Luffy must not only reach the final island, but go a step beyond Roger in taking that which is learned and using it to 'save the world.'
I do not believe the True History reveals something so simplistic as 'The 20 Kings were bad guys who did something bad to the Lost Civilization.' That wouldn't really be reflective of the nuanced manner in which Oda writes conflict. I don't believe the Lost Civilization were 'good' guys, nor do I believe the D's to be simply the survivors. We know that the Nefertari King thought it was the right decision to band together with the other 19 and stand opposed to the Lost Civilization, but decided that it was wrong to ascend to Marijoa. We also know that the Lost Civ. apparently were in possession of or created the Ancient Weapons. Oda tends to write manipulative villains who exploit conflict between two factions to position themselves in a place of power or achieve some other end goal. Examples: Crocodile in Alabasta, Eneru on Skypiea, Teach during Marineford, and Doflamingo on Dressrosa. This would imply that the nature of the conflict during the void century was more complicated. The lengths to which the World Government has gone to hide this truth also implies history is of more immediate concern.
The Minks have a prophecy about the New Dawn. The Celestial Dragons have positioned themselves to live as 'gods' of the world. The D's are said to be the 'enemy of the gods'. The Sea Kings are waiting for a promised day when things will be set right. The world is not in a natural state and the World Government does not represent an corrupt, but inert organization. They actively suppress freedom, are actively discussing a mass 'cleansing' of the world, and are seemingly actinvely working toward something, some unknown end goal.
Joy Boy and his allies failed 800 years ago, but preserved a message for the future. He joined with allies around the world to preserve the message, the truth, for the future. Doflamingo said at the battle of Marineford that it is the victors who decide history. Interesting that this line was spoken by one of the Celestial Dragons. This notion, that the truth can subjectively be decided by the one who claims victory, is refuted by the fact that the losers recorded history on unbreakable stones. They made friends around the world and preserved that message for a successor. The Kozuki Clan crafted the Poneglyphs and they were hidden in remote places around the world, protected by people who passed down a prophecy about the day in the future when the world is put right again.
You have to understand that it is not weapons, military might, or an army of devil fruit users that will decide the outcome of this conflict. None of these things are responsible for the survival of the true history for the past 800 years. Both Roger and Luffy made allies around the world. Their kind nature, desire to protect those they care about even when it means selflessly putting their own lives on the line, and desire for freedom are their greatest strengths. They are the reason Roger was able to reach Laugh Tale and the reason Luffy will be able to do the same. It was through the combined effort of many people around the world. From this, I think it's possible to gain some insight into who 'Joy Boy' was and why he left One Piece on the most unreachable place in the world. He was likely someone much like Luffy and Roger in nature. Clearly he had allies around the world. The Kozuki Clan crafted the poneglyphs to record the message for the future. An unfulfilled promised was made to the mermaid princess. Civilizations around the world including those allied with the 20 Kings (Alabasta) agreed to hide the Poneglyphs. The Sea Kings protect the Grand Line, residing in the Calm Belts, ensuring that the only way to access Laugh Tale is through sailing all the way along the Grand Line. The Road Poneglyphs were protected by ancient societies also known to be longstanding allies who all have similar prophecies about the future. There is a reason that Joy Boy accepted defeat and decided to preserve history. "Keep the truth in your hearts. Keep your mouths silent. We are the ones who weave history." The truth cannot be spoken until the promised day. Roger knew this to be true and simply passed the message along. He and Whitebeard thought it would be Ace to step in and take up his mantle, but it will be Luffy. It was only through these bonds of friendship that the message has been protected and passed along through time - that the losers were able to decide history. It is only through this same 'power' that one can overcome the foe that awaits after One Piece is discovered. The Revolutionaries cannot do it alone. The good Marines can't do it alone. Teach absolutely cannot do it alone. It is not weapons alone that win wars. The preservation of truth has been paramount.
Joy Boy saw that the sun was setting on his age. He hid One Piece in the most unreachable of places. His friends agreed to protect the path to this treasure. Allies around the world hid messages pointing the way. He understood that only a person who could make allies of many people could ever bring the light of the new dawn. Thus the treasure was hidden in a place that only one who possessed this requisite quality could reach. Roger was the first one in 800 years to arrive - and yet he discovered that the time was not right. So he passed the message along. Whitebeard did the same. But now the time is right. Ten years for Shirahoshi to be born, 15 to grow. The two monarchs must meet. Fishman Island must be destroyed. The idea that the final war will just be a chaotic free-for-all isn't really in keeping with the nature of the larger conflict in One Piece. This isn't about who gets to become Pirate King, but what the person who becomes Pirate King ultimately must do with the treasure once it is discovered.
Even Bink's Sake alludes to this. The final two verses:
Going to deliver Bink's Sake!
Today, and tomorrow, our dreams through the night!
Waving our goodbyes, we'll never meet again
But don't look so down for tomorrow night the moon will also rise
Going to deliver Bink's Sake!
Let's all sing with a Don! A song of the waves
Doesn't matter who you are, someday you'll just be bones
Never-ending, ever-wandering, our funny traveling tale!
Now, it's important to note a couple things. Oda has used day/night imagery repeatedly to describe the state of the world. The Minks are waiting for the 'light of the new dawn'. The moon has been alluded to as a beacon to light the way during the night. The 'funny traveling tale' line implies a direct connection to Joy Boy and Laugh Tale. I believe the D's are the descendants of a group of people, of which Joy Boy was a member, maybe the leader. They traveled the world and had many great adventures, making friends wherever they went. The world became unstable. The Lost Civilization came to possess the ancient weapons and represented a threat. The 20 Kings united to oppose them. Joy Boy (and friends) tried, and failed, to stop what was happening. So they preserved the message for the future They became the enemies of the new gods who came to rule the world from Marijoa and were branded'devils'. They took the name D. as the secret symbol of their fellowship, passed down through the generations. It means 'devil', but also looks like a smile and a half-moon. (Like the Half-Moon Vanished Behind the Clouds….). The Straw Hats are united by their belief in the pursuit of impossible dreams, all of which are seemingly increasingly connected to this core conflict. Teach has never slept.
I could speculate further about the nature of this conflict - but keep in mind, I want to distinguish these ideas as not really grounded. They're pure speculation
The Red Line is an aberration in the One Piece world where reside the Celestial Dragons and Imu, whose name is the reverse of Umi (Sea). There's the Grand Line, bordered by calm belts, also strange knowing that the sea kings are sentient beings, seemingly protecting One Piece and ensuring it can only be reached by sailing the entire world. The only way to cross the calm belt is using seastone, found only on Wano, the home of the Kozuki who crafted the Poneglyphs. Then there are the 'Devil' fruit; anyone who consumes them is 'cursed' by the sea. It's almost as if there are two powers in conflict with one another - the power of the sea, of true freedom, and those who wish to control it;
I was going to include my thoughts on who/what Imu might be, but that is beyond the scope of this post. I do not deny Teach will be an antagonist to Luffy, nor that he will be involved in the final conflict. But I do not believe it will be a 'free for all'. The three weapons are not all things to be possessed. Pluton is a ship residing somewhere in the world and even should it be built, Franky has the plans memorized to construct another and it is believed that Robin is needed to awaken the original. Poseidon is a person - the mermaid princess - not someone to be possessed. We don't know anything about Uranus yet. I get the distinct sense the three work together and likely were separated and protected for good reason.