Personally, I'd like Hope and Vanille!
I'd like them to take the NON-numerical FF games, so they have a chance to shine. Final Fantasy Crystal Chronicles, Chocobo's Dungeon, the Crystal Bearers, etc.
a bunch of spin off characters. Although hope would be good. Maybe the guy from FFXIV?
And now we fight like men! and women! and women who dress like men! Gilgamesh
Red Mage: "Job?! Risks?! You're a mass-murdering psycho!"
Black Mage: "..."
Black Mage: "If you think about it, that's really their problem."8-bit theater chaos FTW
Astos uses Rub and a physical attacks for BRV attacks. Rub takes a long time to charge and is slow and inaccurate, but can deal instant Break. His HP attacks are slow physical strikes.
Minwu uses healing spells and status buffs for his BRV attacks. A version of Ultima functions as his HP attack.
Xande has Blizzaga and Thundaga for BRV, and Quake and Meteor for HP.
Rydia uses Firaga and the Chocobo summon for BRV, and summons Titan and Bahamut for HP.
Enuo hits you with his staff for BRV attacks and casts Grand Cross and Dimension Zero for HP.
Mog uses dances for both BRV and HP attacks.
Rinoa shoots her blaster edge at the enemy and launches Angelo at your enemy for BRV. For HP, she uses Meteor for an HP attack, in addition to using Invincible Moon to make you briefly invincible.
Quina uses blue magic for both BRV and HP attacks.
Auron uses the Dragon Fang and Shooting Star Bushido techniques for BRV attacks and Banishing Blade and Tornado for HP.
Also, maybe some people from the spinoffs.
Eald'narche uses spells and abilities from his boss fight.
Balthier shoots at the opponent with his gun for BRV and uses his Quickenings for HP.
Sazh shoots at the enemy or gives you buffs for BRV and uses Cold Blood and Firaga for HP.
Celes, Locke, Ghestal would be nice :P
Did I miss something? I thought Aerith would be the only Assist-only character. Personally I hope it stays that way. I would rather have only a few new playable characters than a huge slew of AO characters. Besides, I don't think they're done with new characters. Even if they aren't in the game to start with, I have a feeling we'll see some add-on content.
There there be giants.
At last, the triumphant return of my old avatar!
Maybe Darkknight or Dragoon from III. Personally I'd love to see Sazh as an assist from XIII even though I personally think all of the characters in XIII would be great playable. Even though I'd perfer Auron and Rikku as playable characters from X they'd also make great assist. Vivi from IX would be pretty cool. If Aerith could make assist I'm pretty sure Rinoa would make a good one too, but these are just my opinions
-FFI: White Mage or Princess Sarah.
-FFII: Anyone. Borghen for the lulz...?
-FFIII: Refia, Desh or Xande.
-FFIV: Rydia, Tellah, or Yang.
-FFIV TAY: Ceodore or Maenad. (or both)
-FFV: Lenna or Faris.
-FFVI: Setzer, Relm, Edgar, Gao, Cyan or Sabin.
-FFVII: Yuffie or Cid.
-FFVIII: Seifer, Rinoa or Quistis.
-FFXII: Dr.Cid, Ashe or Balthier.
There's going to be more Assist-only characters, that's for sure. Healers can buff your character or just cast protective spells, like...Protect to reduce BRV damage taken and so on.