Even if Zexion WAS alive, it has clearly been stated numerous times that fighting is not his style. Plus, his Title in the Proof of Existence was "The Cloaked Schemer." Schemer is traditionally associated with manipulation and lying. Zexion's only weapon of note is his wit and... well, scheming. Anyone he sees as a threat, he tries to eliminate with psycological warfare. Perhaps Zexion has no physical weapon as such - he merely uses his skill with words and psychology. And, above all, to completely disprove this whole Zexion theorem - HE. HAS. BEEN. ELIMINATED. He's dead. "He's gone, he's killed, he's dead", as said the Nurse in Romeo and Juliet.
Other possible foes:
Roxas - No. I can't see it happening. Unless they make that whole dream fight thing playable... no, can't see it happening.
Zexion - Effectively disproved.
Nomura - That would kick arse-biscuits. But no.
Anyone Else Mentioned In The 10 Pages I Couldn't Be Bothered To Read: No. Because I say so.
I have my own suggestion... (Dramatic drum roll...) KING MICKEY!!! Dun dun duuuuuunnn! Like a fun battle, but harder than hell. You'd barely be able to hit him! If someone's already mentioned this: Too bad. I take it for my own! Mwhahahhaha! >=D.