Those are some interesting ideas you've got there, but I wonder... Could we maybe extend the list a little? Like let's say:
FFI- There may not be any specifically named heroes here, so maybe just classes could be added there (Like, say... Black Mage?)
FFII- Leon would definetely make a good addition to the list. Who wouldn't like another dark knight?
FFIII- That mystery character wielding the two swords would be good. Y'know, the one on the cover?
FFIV- I can't really think of anything, Kain was pretty much all I could think of.
FFV- Maybe Faris/Sarisa? A pirate should always be welcome.
FFVI- Locke, he's just so cool.
FFVII- Definetely NOT Zack, seeing as Cloud's already in the mix, and if Zack was added, that'd be nothing new, really. I think Vincent would be a good choice.
FFVIII- Quistis. I don't think I need an explanation, she's hot, and she's got a whip.
FFIX- ... I got nothing...
FFX- Auron. He could be a challenge for Sephiroth (Masamune wielders, anyone?). Or better yet, Rikku, to add some cheerfulness to all the depression!
FFXI- I'm gonna completely skip this one.
FFXII- NOT Vaan, because I already mentioned a pirate, and NOT Basch, because his brother's obviously gonna be there.
FFXIII/Versus XIII- Lightning was a good choice, but instead of Snow Villiers, how 'bout Noctis? (That awesome guy that fights with a whole buncha swords flying around him.)
... how's that? It's probably no good, but I posted it anyways.