I'd like to see Quina. Just because it would be hilarious to play as him/her and to see peoples reactions.
Another one I'd like to see even though it would never happen, would be a mixture of the Final Fantasy tactics class. For example, you could play as a squire, and you could summon different jobs to assist or you could "job change" into them and have different combos.
Supers could be assist from the main characters. Such as Rafa and Malak as level one super, and they would use their signature spells. It would hit randomly on the screen just like in the game.
Second super could be a combo attack between the knights such as Agrias, Cid, Meliadoul, Beowulf, and Cloud (?), and it could hit in an circular area around you.
Third super could unleash the Zodiac monsters or Ramza and Delita.
The last one I can think of is Lulu's dolls. Lulu could be in the background like Pokemon Trainer in Super Smash Bros Brawl, manipulating the dolls. Supers could be magic spam.
#1 above all else, Gilgamesh. He'd fit here with his dimension hopping. He could rep the series as a whole with his fake swords too. So we could have your Clouds and Lightnings and Squalls.
After that everything is just unfeasable. Just my dream characters for this. Shadow, Gogo, Celes, Mog...Edge, Kain, Cecil, Minwu (lol), Vivi, Amarant...the list goes on. Setzer, Galuf, And now i'm just rambling >___>
Who would win, Jedi Charizard or Jedi Jigglypuff? "I'm a bagel whore" -blazin640