Of course. My true love when it comes to RPGs is just Final Fantasy VII, that's all.
If we could have other heroes from IV, I'd want (and this is me trying to think realistically too, not just fanservice favorites. hard to do and I'll probably fail at the realistic choices thing, but hey) someone like Rubicante as an honorable combatant for the Warriors of Chaos.
It's hard to think of another two characters important enough for the original Final Fantasy asides just giving us another Warrior of Light with a different class. In Final Fantasy II, I wouldn't mind someone like Maria or something.
Final Fantasy III, I do not know a lot about so...
Final Fantasy V, I have a good feeling that we could get Gilgamesh. I believe it was said by Nomura that if we got a sequel, Sazh, Gilgamesh, and Kain would all be in the next one. So far we've got Kain (and Sazh could be a second hero for the XIII team unless they truly went with Lightning in the end, of which I approve. love them all though)
Final Fantasy VI is a tougher one, but... Locke and Celes have good shots at being another hero for Cosmos if we get more. As for villains, I can't truly think of anyone who is exactly good enough asides Kefka.
VIII; Seifer as a Warrior of Chaos is pretty obvious.
IX; Vivi and a Black Waltz.
X; Yuna and Seymour. Auron for a candidate too.
XI; (don't know squat about lol) ....
XII; Vaan, Ashe, Vayne, Balthier. Prime choices right there
XIII; Lightning, Sazh, Cid Raines, Galenth Dysley
Versus XIII; Noctis Lucis Caelum
Final Fantasy XIV; whoever stands out I guess, kinda like Shantotto in XI.
Doubt we'll get all of these game series' in (and by that, I mean Versus XIII and Final Fantasy XIV), but it's still worth mentioning. Some of my picks imho.