I'm certainly wrong on several of these, since Oda's going to introduce important elements in the future which I don't know about, such as what exactly the One Piece is or why it got its name. I should note most of these theories are not originally mine, too.
One Piece is definitely not going to end with Luffy finding One Piece and becoming the Pirate King. It's been said that the claiming of One Piece will cause a great war, so the obvious interpretation of that is that it'll be the war to destroy the World Government. Luffy'll face and defeat Blackbeard at Raftel to claim One Piece, and for whatever reason, this'll lead him to decide to destroy the WG (the Rio Poneglyph is obviously going to play a role in this).
So, like how Whitebeard called in every ally he has to fight against the WG in Marineford, Luffy will call in every one of his friends to join him, though it'll be even larger scale than Marineford. Alabasta, Dressrosa, Drum/Sakura, Fishman Island, probably future islands like Wano and Elbaf, and maybe even kingdoms like Prodence – they'll throw in the full might of their armies to join the final Strawhat Fleet. In fact, I expect every single one of the Strawhat allies to be there, ranging from Dorry and Brogy, to the current Colloseum fighters, to the guys who trained the Strawhats like Heracles, to Lola, to maybe even Foxy. If you think Dressrosa involves too many characters, the finale is going to wreck you.
The Revolutionaries will certainly join, possibly led by Sabo, as are some "defected" Marines who're shocked by whatever was revealed by the One Piece, led by Coby and Smoker. The "old generation" might appear, like the former Whitebeard crew and Raleigh, but I doubt it.
I expect the WG and the Strawhat Fleet to each have an Ancient Weapon, to up the stakes. The Strawhats are going to have Shirahoshi/Poseidon, while the WG will have Uranus or Pluton, unknown at the moment.
Now, there's the matter of how the Strawhats will get to the final battle at Mariejois/Fishman Island/New Marine HQ. After getting One Piece, they're almost certainly going to jump over Reverse Mountain and meet Laboon and Crocus. They can't sail backwards through the New World, and it would be boring to resail Paradise.
Now, the Red Line. We know absolutely nothing about it, other than that Mariejois is on it. Why's Oda so unrevealing about what goes on the Red Line, when he reveals so much trivia about random islands? The best conclusion I can make is that the Strawhats will traverse the Red Line at some point, over land.
Mariejois is almost certainly going to be destroyed, and more speculatively, so will the entire section of the Red Line there, as that theory goes, creating All Blue. While the original theory says that the Grand Line will be erased by this, I don't think it wlll – rather, my guess is that it'll become an oceanic version of Reverse Mountain, a second entrance to the Grand Line.
The fall of the WG and the creation of the All Blue entrance will encourage more people to go to the Grand Line and become pirates, kickstarting the Second Golden Age of Pirates, inspired by Luffy instead of Roger. The epilogue will likely cover this.
So there.
