The amount of time a character has been in the story is not a good metric to determine which character is the final villain.
"These things cannot be stopped. An inherited strength of will. One's dreams. The ebb and flow of the ages. As long as people hunger for freedom, these things will exist" - Gol D. Roger
This quote accompanied Dragon's introduction at the start of chapter 100, The Legend Begins.
It should be apparent at this point that this is not a story about piracy. The story takes place in a world of pirates, but it is about a much broader and more universal theme.
Luffy believes in dreams and seeks true freedom. One Piece and the title of 'Pirate King' are means to that end. They are not the end in and of themselves. Teach believes in those same ideals. Only one can inherit the treasure and the title. It has become clear over the past 980 chapters that the treasure represents more than the fulfillment of selfish desire. To discover One Piece is to inherit the burden of history. It is to inherit the responsibility of completing that which Roger, the first to discover the treasure, and Joy Boy, the one who left it 800 years ago, could not or did not.
Teach is not Luffy's thematic antithesis. His speech when the two meet in Mock Town underscores the idea that the two share largely the same beliefs. What differs is their moral character. In simple terms, both are dreamers who openly pursue their goals in waking hours to make them reality, but Luffy is good and Teach is evil.
It is my honest belief that one with Teach's character is not capable of using One Piece for its intended purpose. I belief its placement at the end of the Grand Line and the method required to determine its location through use of the Road Poneglyphs, coupled with the fact that Roger did not widely disseminate that which he learned, implies that the journey itself is a necessary aspect of the treasure. It seems designed to filter for a person capable of making allies and friends across the world. To court the favor of the Kozuki Clan, capable of translating the Poneglyphs, and the respect of those who guard the Poneglyphs is the only way to learn the True History and reach the final island. Possessing the treasure without having undergone the necessary purgation of selfish desire required by such a journey renders the person who yields the ultimate prize unable to use its power to restore and enrich the wider world. Luffy is the person Roger is waiting for. It wasn't Ace. It's not Teach.
For the final conflict, there needs to be a contrast of ideals. The world of One Piece is in a state of night. The sun set 800 years ago when the void century ended and a new power took control of the world. The whole world groans from whatever causes this imbalance and waits for a promised day in the future when the world will be set right. The Sea Kings are waiting. Zunesha is waiting. The Minks, the Kozuki, the Fishmen, all entrusted with Poneglyphs are waiting and their people all have symbolism of the moon guiding the way in the long night. Dreams are the vessels which conduct the individual through sleep to the dawn. The Straw Hats are all guided by 'impossible' dreams. I believe it is becoming clearer that these dreams all directly relate to the singular goal represented by reaching One Piece.
The title of 'Pirate King' was not one Roger claimed for himself. It was declared by the world's papers. Roger didn't do the things he did to become Pirate King. He was declared such because of the things he accomplished. Luffy's seeking the title, yes, but for a purpose. That purpose, to achieve true freedom, is his ultimate motivating desire. The title itself is meaningless. Like, as a point of comparison, Naruto wanted to be Hokage in his series and accomplished that goal in the final chapter, but what that title represented was a responsibility to be the protector of his people. It wasn't vanity. It wasn't a trophy to mark the end of his path. It's the start of a whole new path. The path of the hero is a circle, the final leg of which is return. The ultimate reward or treasure comes after the hero's descent and transformation - a refining process of trial and tribulation. Like Prometheus who stole fire from the gods, the person who discovers power, knowledge, truth, etc. - it's ultimately supposed to be for the betterment or transformation of the world at large. What Luffy must ultimately learn, as did Roger, as Whitebeard declared at Marineford, is that learning the truth of the world, possessing One Piece, and becoming Pirate King is not the end of the path. It's not the finish line. It means inheriting a greater responsibility to the world.
The world in One Piece is already bad, otherwise One Piece would not need to exist. The Poneglyphs and the path to reach One Piece would not need to exist. The world would not be waiting for a hero. Teach cannot make the world worse in a meaningful way to the narrative in my opinion. As stated above, I do not believe he's capable of changing the world. We do not understand why yet, but it's not possible without One Piece. The path to One Piece was designed to filter out a person like Teach and allow passage of a person like Luffy. Teach is Luffy's shadow - the dark side of his desire. He's a moral opposite, not a thematic opposite. The final antagonist should represent Luffy's thematic antithesis - one who stands for an end of dreams, a world without freedom, without idealism, without the ability to pursue a goal. A world of totalitarian control where the will of a singular individual is exerted over all others. That is not the world Teach represents. Even those who believe he's the final villain have sort of attached on this idea that Teach is somehow an 'agent of chaos.' I disagree with this assessment of his character, but 'chaos' is not antithetical to Luffy's ideals. It doesn't represent a world without freedom. It's a world with excessive freedom. It's Luffy's ideal untempered by good governance. Others have assumed Teach wants to succeed where Rocks failed. I do believe this is a more likely motivating desire. That said, I don't believe Teach will succeed in this ambition. Whether Teach wants to be Pirate King or World King, he seeks power for a purpose. We don't understand Teach's motivating desire yet, but we do know that he shares Luffy's belief in dreams. Let's say Teach becomes ruler of the world. Has he expressed his desire to stamp out dreams and create a world where no one can live freely? I think not. Not Luffy's thematic opposite. Let's say he becomes the king of a chaotic world. Again, not Luffy's thematic opposite. Let's say he just takes the place of whoever currently rules the world and does exactly what they're doing. Doesn't make narrative sense. He either did what only should be able to be done with One Piece without One Piece or he discovered One Piece instead of Luffy, depriving Luffy and the readers of the experience of the Straw Hats discovering Laugh Tale. Sure, Luffy could go beat up Teach and take the title of Pirate King away from him at the very end of the series, but it would be essentially meaningless. The whole journey has been leading up to something. As I said, the King has a responsibility. If Luffy has already saved the world when he inherits the title, it's worthless. It represents only fulfillment of selfish desire, not the responsibility to use inherited will and power for the betterment of others.
I also wonder in what ways people believe Teach could make the world appreciably worse by becoming 'world king'? The current power has been in place for 800 years. He certainly will not have a longer tenure. Barring a long time skip where Teach rules the world for several years, I struggle to imagine what he could meaningfully accomplish.
What has been established at this point is that the world is waiting for a savior and One Piece is the key. Discovery of the treasure means the start of a global conflict. Whatever is happening began many centuries ago. The void century starts 900 years in the past, ends 800 years in the past with the rise of the World Government. Zunesha's curse was initiated 1000 years ago. The treasure was left in the most unreachable place in the world for a purpose and the path to reach it was designed as a great filter - to test and purify the person who would acquire the treasure and learn the truth of the world. Roger completed the journey, but was too early. The world itself wasn't ready yet. The pieces weren't in place 25 years ago. The world declared him 'Pirate King' for his accomplishment. He used his notoriety, the influence granted to him through the successful completion of his journey, to spur on a successor. The World Government has been struggling to stamp down the age of Piracy because, above all else, they do not want Roger's successor to discover the treasure and learn the truth of the world. I do not see a scenario here where Teach fits as a larger threat, more dangerous than the power that One Piece itself was meant to stop, or a greater thematic antithesis to Luffy's motivating desire. The world does not have freedom. If it did, One Piece wouldn't exists, Roger wouldn't have delivered the quote at the opening of this post, and people around the world would not be waiting for The New Dawn, the Path Toward the Sun, the Promised day, etc.
–- Update From New Post Merge ---
@Claudio:
I vaguely remember that bridge and that it has been under construction for centuries. Can you tell me what chapter it is mentioned?
Imu obviously doesn't have full reign over the world and he/she rules in secret. Why do you think that is? It doesn't look to me like someone who wants full reign over the world.
Not having something is not the same thing as not wanting something. What about the World Government screams 'we don't want total control'?
The fact that they don't have what they want means that they are not a stagnant, but active force.
It also speaks to the fact that no one who seeks power for its own sake is ever satisfied. Those who seek power and acquire it still desire more power. Unlimited power.