@S.C.:
I'll predict the roster size to be a minimum of 35 to a maximum of 40.
! 1. Ace, 2. Akainu, 3. Aokiji, 4. Bartolomeo, 5. Blackbeard, 6. Brook, 7. Buggy, 8. Burgess, 9. Cavendish, 10. Chopper, 11. Crocodile, 12. Doflamingo, 13. Enel, 14. Franky, 15. Fujitora, 16. Garp, 17. Hancock, 18. Ivankov, 19. Jinbei, 20. Kidd, 21. Kizaru, 22. Kuma, 23. Law, 24. Lucci, 25. Luffy, 26. Magellan, 27. Marco, 28. Mihawk, 29. Moriah, 30. Nami, 31. Robin, 32. Sabo, 33. Sai, 34. Sanji, 35. Shanks, 36. Smoker, 37. Usopp, 38. Tashigi, 39. Whitebeard, 40. Zoro
Okay so the ones I didn't have (Burgess, Cavendish, Garp, Ivankov, Kidd, Magellan, Marco, Moriah, Sai, Shanks, Tashigi) probably most of them will be in the game, but I don't they are essential.
Obviously I want them all too, and I think this is more accurate now that the 3v3 aspect has been revealed.
But before that I would have thought that these characters either haven't done that much 'fighting-wise' (despite being important) or are just too minor a character to make it into the game
Ivankov, Magellan, and Tashigi - probably should have been on my list, though Tashigi has neither Devil Fruit powers or much Haki
Burgess, Cavendish, Garp, Kidd, Marco, and Sai - We've probably seen about the same amount of moves and fights from these characters as Bartolomeo, and he is in, so all of these can go either way. It's not that they can't make up a moveset, its just these have less source material to chose from. And most of them were in pirate warriors. Marco probably has a pretty good chance
Moriah just has a bad history with games, Shanks on the other hand has a great one
Caesar clown is another one that'll probably make it in