Rinoa should be in it! Squall/Leon received that letter from her in the end (and she's in hardly any other cameos) so....yeah! >:O
Tifa I don't care much for as she's over saturated just like most of the VII gang, Lightning will definitely be in it because...it's Square...and I'd like to see a VI character that's not Terra (preferably Celes or Locke) but I know that's highly unlikely.