Well, if they're going the route of doing the whole Org XIII, then here would be my prediction:
I. Master Xehanort – While the old man's moveset was never impressive back in Birth by Sleep, it would be super-badass if they gave MX a moveset akin to No Heart from BBSFM, seeing as how No Heart is meant to represent Xehanort when he was in his prime.
II. Xemnas – Obviously, they've gotta have him go Final Form Xemnas on us. That's just easy.
III. Ansem, Seeker of Darkness – Here, Ansem could definitely incorporate more of his moves from the final battle in KH1.
IV. Young Xehanort – Of course the time machine trick from DDD would be cool, and let's just face it, he could go complete Mysterious Figure on us and have all of the crazy moves from that fight. He's practically begging to be an amped up superboss in this game.
V. Terra-Xehanort – This one is trickier, because there aren't very many incarnations of this boss in the Kingdom Hearts series. A lot of the moves he already has pretty much cover the extent of his abilities in KHBBS, so I'm not sure what they could do with him here. Maybe they could just give him some more Lingering Will-esque moves like the Keyblade bazooka that he had in BBS.
VI. Vanitas – Well, it looks like they're going with Vanitas' Lingering Spirit abilities, which is a-okay by me.
VII. Xigbar – Xigbar also is tricky, but it seems like they're going back to playing more with his sharpshooting minigames. Who knows, they might also do more of those trippy arena-alterations he did in KH2, being that his element is space and all.
VIII. Saïx – Not much here except they could make him stronger and give him more invincibility frames. He was by far the easiest boss in the gauntlet.
IX. Luxord – Definitely do more of the time-based games like in KH2.
X. Marluxia – Now, here's some possibilities. Of course, they could bring back the death counter and more of his moveset from KH2, but he seems to already have a fair number of moves from that game. Alternatively, they could bring in the big machine he used from Chain of Memories and do some crazy shit with that, but my guess is that's pretty unlikely.
XI. Larxene – Cloning. Bitch can get some clones.
XII. Dark Riku – This one I'm at a bit of a loss for. I don't feel it would be right to put in any moves Riku uses past KHCOM since he's supposed to have moved past the darkness by that point, so barring that I can't really think of what other moves or forms he could draw from that they didn't already cover in his main story fight. Got me on that one.
XIII. Xion – Okay, here's one they could really spruce up. Her fight in the story was pathetic. They could give her all that crazy armor-form shit from Days, and that's all they'd need. Plenty to work with there.
That's all I've got. Let me know if any of you think of anything else.