I doubt there would be another Dissidia, though I would love for there to be one, but if they were to make a third one then I would like to see:
FFII: Maria as an assist. It would be interesting to see her in 3D. She's pretty much the forgotten female character of the FF series.
FFIV: Maybe Rosa? If you're going for an Aerith like assist. For offense then maybe Rydia. But I would much rather see Rydia in the game as a new character, rather than assist.
FFVI: Locke (if he weren't a new character) for offensive attacks.
FFVIII: Rinoa, or Zell
FFIX: Garnet for healing. Vivi should be a new character, so I wouldn't want him as just an assist.
FFX: Auron needs to be in the game as a new character. So maybe add Wakka as an assist? He could do various status and melee attacks.