I'd love to see Seifer in the next Dissidia... but what I'd really like to see is Caius.
I also wouldn't mind Snow or Sazh... And the usual: Yuffie, Vincent, Balthier etc.
And I'd reaaally really love it if anyone from Type-0 joined Cosmos. Anyone really. One of the Kunagiri brothers, Rem, Sice, King, I don't care. Even Nine or Queen.
I'm surprised she hasn't been mentioned, at all, but I'd love to see Valvalis/Barbariccia. I don't really dig the "assist" much, and I think all characters should be playable. But, due to the fact that the assist is there, and for a valid reason..! I guess Valvalis would be more of an assist, though I'd prefer to see her playable.
**ETA**
And yes, I realize, she's already a summon. But I don't like that. XD It should be changed.