It's been done many times and I've have this theory up for a while on other sites. So far, this is how I piece the "One Piece" story together as far as The Void Century, The Will Of The D., The Ancient Weapons and all that other good stuff goes.
The Will of the 
We know it consists of people that possess the middle initial D. They're reckless, carefree, and (with the exception of Blackbeard) completely fearless. Their will is indomitable, they're persistent, and they …never seem to wanna give up. All of the known D.'s that passed on have smiled or laughed as they died. It's been speculated that they come to terms with their end and leave this life peacefully. They also follow a code of moral justice and also believe in a form of destiny and/or fate, Blackbeard included. I personally don't think that they're related to each other because Jaguar D. Saul was a Giant. I think they all descended from a group of people in the Void Century.
Void Century:
I think that in the Void Century, the carriers of the D. initial were missionaries of some kind that were sent out from the Ancient Kingdom to send a word out about a plan that was thought of by the actual king of the Ancient Kingdom. One of the guys I was talking to about this said that he thinks that the king of the Ancient Kingdom was "Joy Boy". I agree with him. But the group went out to spread the news of a plan that Joy Boy may have called, "One Piece". We came to the conclusion that "One Piece" was possibly a plan to unite all 4 Seas. The seas are divided by the Red Line and Fishman Island. The King told his missionaries to deliver his proposal to all the territories of each sea. The proposal was for Reverse Mountain and Fishman Island to be destroyed so people can sail with ease from sea to sea for easier exploration and also union among each sea. The 4 seas 'would' be a single sea if Fishman Island and Reverse Mountain weren't there. I think that's the purpose of the Ancient weapons tho. But this in turn would create the "All Blue". We know that that is Sanji's dream. This would also lead to extra freedom to sail on the sea, possibly even easier evasion from the Marine forces. Luffy said the one with the most freedom is he that could be Pirate King, and of course finding "One Piece". This process is also how Brook would get back to Laboon. But anyways, the carriers of the D. sent out the word. I feel that some agreed. I'll go as far to say that the promise between Joy Boy and "Poseidon" of the Void Century was that if she agreed to let Fishman Island be destroyed, there would have to be a way for her people to be moved to a safe location. Joy Boy made her a promise that he would help them get to the surface "to live under the sun". I'm guessing that's the Noah's true purpose. She could just use her powers and have the Sea Kings pull Noah to safety. But the 20 nations that didn't agree formed together to topple the Ancient Kingdom. I'm guessing Joy Boy knew the kingdom would fall and the remaining carriers of the D. initial in that area fought against the nations that tried to topple the Ancient Kingdom. Also foreseeing the fall of his kingdom, he had the plans of "One Piece" inscribed on the poneglyphs by the missionaries in different and secluded areas around the Grand Line. I'm guessing that Joy Boy had the travelling missionaries settle in each of the seas to where they traveled, if they weren't killed for the proposal I assume. I also feel that's the reason why the D's aren't found in just one sea. The kingdom fell and all traces of the prosperity that was the Ancient Kingdom was wiped out and/or hidden (minus the poneglyphs). This gave rise to the 20 nation's ideal protectorate of justice: "The World Government".
Current dreams to be fulfilled (Straw Hats):
Luffy: "Pirate King" is the one who finds "One Piece" and has the most freedom to sail the sea at his or her own desire. He'll also be able to return the Straw Hat to Shanks.
Nami: With the seas being united as on entity, it would be more than possible to create a map of the entire world.
Sanji: Aside from finally seeing a mermaid, he'll finally have found the "All Blue", fulfilling both His and "Red Leg" Zeff's dream.
Brook: Reunites with Laboon at the mouth of Reverse Moutain. Prepare yourself for "Bink's Sake", Brook's trademark song.
Robin: She will know the "True History" and will have found each of the poneglyphs needed to piece together the events of the Void Century. She also fulfills the dream of her mom and the other Ohara scholars.
Usopp: An accomplished and well-known sniper(Warrior of the Sea) that even his dad wouldn't be able to best.
Franky: He will have made his Ultimate ship/weapon, seeing that he has the plans to Pluton memorized and is slowly modifying the 1000 Sunny to travel across the seas.
Chopper: To be a great doctor
Zoro: World's greatest swordsman. By the time they reach Raftel, Zoro will be ready to battle Mihawk.
Ancient Weapons Thoughts:
In relation to the theory provided, I feel each Ancient Weapon has a purpose of use...
Pluton: There will be quite a bit of turbulence and whirlpools that would cause damage to even the sea creatures in the sea at that depth damage if Fishman Island were to be destroyed. I feel that Pluton is a ship that can not only, destroy Fishman Island, but withstand the turbulence of the sea in the process of doing so.
Posiedon: The Mermaid that possesses the ability to control Sea Kings. She can summon a Sea King of any given sized. I feel like she'd use her ability to destroy Reverse Mountain. Since her and the Sea Kings are "Beings of the Sea", it will allow them to stand against heavy currents that are the size of the wave that almost engulfed the sea train back in the Water 7 arc, as well as whirlpools and any other types of major disturbance in the sea.
Uranus: "God of the Sky" in Roman/Greek mythology. This weapon or being may possess the power to wipe out the sky kingdom of Marejois. It's been specified by numerous fans that Eneru/Enel may have something to do with that, but whatever the case may be, it could possibly be a weapon capable of destroying that kingdom.
It's been said that the World Government is in possession of something that is equal in power to the Ancient Weapons. In Z's ambition, a newer One Piece movie, there were weapons that were strong enough to destroy the entire New World. Idk for sure if those are the weapons that the Government is talking about in particular, especially seeing that the movies are non-canon, but it's a possibility.
Blackbeard:
I'm not exactly sure what the true role of Blackbeard may be, but since he was a part of Whitebeard's crew for the duration of approx 20 years to get the Yami Yami no Mi before he actually attained it, implies that he's a patient and calculating individual. He was willing to kill someone that he had been sailing the sees with for nearly 2 decades without even batting an eye. He also was more than happy to battle and turn over a former commander of the crew he served with as well (Ace). He may battle Luffy before the plan of "One Piece" can be carried out. He may also make it to Raftel along side the Straw Hats and battle them to atually make it on the island to find out the plan and either destroy it or carry it out. I'm not exactly sure of what he could be planning. Oda said that Blackbeard was his favorite pirate, so it would be quite a twist if Blackbeard wound up as the Pirate King. I definitely don't think that he was given the name of the most infamous pirate in history for no reason.
Side Notes, Storyline Hints, and Useless Facts:
- I think Doflamingo may be a World Noble, kin, or decendant of the Celestial Dragons. I could be wrong tho.
- While the Straw Hats were on Fishman Island, Madam Shirley gave predictions that were never wrong. She did say that Luffy WOULD eventually destroy Fishman Island. That's just a thought though...
- Laboon was trying to destroy Reverse mountain as the Straw Hats entered the Grand Line to get back to his crew. This also speculates that if Reverse mountain was destroyed, it would have been possible for Laboon to get home and/or to the Rumbar Pirates location.
- Also, atop Reverse Mountain, you can see 4 currents from each of the 4 seas convering at a single spot at the top of the mountain.
- If Luffy was in the real world, Oda said he would be Brazilian.
- If Zoro was in the real world, Oda said he would be Japanese.
- If Usopp was in the real world, Oda said he would be African.
- If Nami was in the real world, Oda said she would be Swedish.
- If Sanji was in the real world, Oda said that he would be French.
- If Chopper was in the real world, Oda said that he would be Canadian.
- If Robin was in the real world, Oda said that she would be Russian.
- If Franky was in the real world, Oda said that he would be American.
- If Brook was in the real world, Oda said the he would be Austrian.
