I gotta admit, i was thinking about this all day during lectures and I think I'd really like to see some Sqaure-Enix characters make it in.
I don't even really play JRPGs anymore but the SC series would pretty much be a perfect fit for someone like Lightning, Cloud, and Sephiroth.
I know it sounds pretty weeboo-gay but some of the older Final Fantasy games were games I played growing up as a kid so there is that bit of nostaglia there.
Alot of characters don't fit. Kratos fits better in MK in my opinion since he is so violent and his games are filled with gore. The star wars characters, while cool, I didn't feel really fit considering they have LIGHTSABERS but I understand that their presence probably sold a bunch of games to that demographic.
I guess, Final Fantasy, whatever you think of it in the end, (and i'm not even that big of a fan) seems to make a good fit for SC. Dudes and Chicks with swords and weapons... I mean look at the Dissidia games for the PSP. I don't play handheld fighting games but I will admit that if it was on a proper home console I'd probably have givin it a shot. The characters seem to transition naturally into a fighting game considering all they do in the rpg games is...fight. lol