Dissidia Final Fantasy is out in North America, and I've been playing waaaaaay too much of it, but here are some of my musings on what would be interesting additional characters.
I'll try and keep the list limited to characters that would be fun to play, but some, er, punching bags may have also made it into the list.
So, here we go, my list:
FINAL FANTASY I
Warmech
...Well, I think he'd be an interesting character.
FINAL FANTASY IV
Yang
There are no monks in Dissidia Final Fantasy. It would be interesting to have a pure martial arts character.
Cain
How can we have no one who can Jump? There are also no Dragoons in Dissidia. And it's a toss-up if Cain would be Cosmos or Chaos.
FINAL FANTASY VI
Edgar
Edgar is a "mechanic" and uses various tools. This could really be an interesting game mechanic.
Setzer
Setzer is a "gambler" - which means that his attacks would all be luck based, I guess. He worked in Kingdom Hearts II, though, so...
Mog
Come on, who doesn't want to try and beat up Sephiroth with a Moogle Dancer?
Relm
Her Ex-Burst would be "crash the game and potentially corrupt your save game."
And including her would mean that you could kill her over and over again.
FINAL FANTASY VII
Tifa
Again, a pure-martial-arts character. And her limit break fits into the Ex-Burst system perfectly - each hit up to Final Heaven is a Bravery hit, then Final Heaven finally does HP.
Reno
You gotta have the Turks. Also, I'm noticing this is the first character who could be fighting for Chaos I've included. I suppose I should give honorable mentions to Tseng, Rude, Elena, and Cissnei.
Hojo
A definite Chaos supporter. Not sure how he'd work - but it'd be interesting, to say the least.
Aerith
If for no other reason to play as Sephiroth and kill her.
Yuffie
See Relm, minus the game crashing.
FINAL FANTASY VIII
Selphie
See Yuffie.
FINAL FANTASY IX
Vivi
Because he's cool, that's why. Just because he'd be a clone of Terra is no reason not to include him.
FINAL FANTASY X
Auron
'Nuff said.