So we know OP will have a final battle. That much seems clear based on the trend of story progression and Oda's own comments about the series.
Asking who will fight whom seems absurd at this point….and it is. We don't know enough about our own main characters let alone all the villains so to venture guesses at who will battle whom is WAAAAAAAAY out there.
Rather the question I'd like to pose is: In what way will the final battle pan out?
Truth be told this most recent arc's finale was along the lines of what I expected from the finale. What with Luffy taking out entire war ships and creating fire and brimstone in the sky.
So what can they do? Oda has been VERY good about not repeating the same scenario twice so we probably will not get another maritime battle quite like the one we just had.
How do you think it might be pulled off and furthermore, with SO many forces introduced, do you think it might be far more complex than just X VS Y?
I mean unless OP beats out Golgo 13, we can't possibly expect Oda to have Luffy battle each Shichibukai one by one. Not only would it be incredibly long, it would also be incredibly boring. Sorry, but it would. Wouldn't YOU want to move the story along even if you yourself were writing it?
We also have Marines up the bung hole and now 4 major pirate captains.
There will not be an arc for every single one of them. There are few things I'd say are impossible in a manga world but that's one of them.
So what kind of action can we expect to see?
If this final struggle occurs at Raftel, might it depend on the geography of said place? With a 10 (or 9 or whatever) member crew, Oda has plenty of Strawhat Crew to go around but it still doesn't add up to each of the strongest of each 'Power' VS one of the crew.
There's going to be some interesting twists he pulls and I'm physically exhausted just THINKING about how he can handle the pressure of dealing with so many characters that, as we all know, are one way or another going to have a very big showdown.