@Laffite:
Here is my theory.
THe last battle will be an all out war.
Pirates vs Revolutionaries vs Marines…
Now, I believe the battle of Ace and Blackbeard leads to Blackbeard joining the Shichibukai because he most likely defeated Fire Fist Ace.
Somewhere at the end(about Raftel maybe)there will be a final confrontation and most of the pirates respect the strawhats and will fight by their side.
Obviously there will be a lot of emotional scenes(perhaps Luffy's previous enemies thinking about how they train so hard ever since they lost to him) and Luffy will fight Blackbeard who has gained so much power as a warlord of the sea.
Not gonna work because of a few reasons:
1. There is no reason for the war to take place anywhere near the end of the Grand Line. The marines got no real reason to go to Raftel, and since WB will kick their butts because the'll make BB shichibukai he doesn't have a reason to go there either as long as the WG doesn't pay. The revolutionaries don't really care about Raftel either, just want the WG to crumble.
2. Strawhats didn't really get alot of pirates that respect them (not anyone who would help them in a fight anyway): Buggys crew hates them, Kuro hates them, Arlongs crew hates them, Don Krieg hates them, Crocodile hates them, Bellamy if alive hates them, Foxy hates them, most of the WG hates them.
Anyone who respects them are people who had to be saved by them in the first place, and of course Shanks crew…......
The war will take place before the final confroation, maybe an arc or so away I think, Ace death\defeated will be the trigger an any allies will be more or les allies by chance.
But yeah, it will involve Pirates, WG, and Revolutionaries. Its just a question who will be on which side.