Yun and/or Yang: They are actually already in SFIV, in a cutscene. There would be no problem introducing them in a new edition of SFIV if you just made some story about it, which Capcom knows how to do. There could be some interesting dialogue between the two and their master, Gen. That, and they were some of the most popular characters in SFIII.

Adon: We already have one returning character from the original SF, why not complete the cast? 

Guy: Guy was my favorite character in the Alpha series, and as I said before, FF characters are awesome.

Ibuki: My second favorite character in III, Ibuki is a unique character in SF, which is why she deserves to come back. There aren't any other ninjas! She was also really fun to play. I'd say, if SFIII characters come in the next version, she'd be likely due to the appearances she's had in Capcom "chibi" titles.

Oro: HE'S GOT ONE ARM.

Ruby Heart: I don't know. Give the chick some reason to exist, not just as some random face in MVC2.

B.B. Hood: Final Fight characters can make the crossover, why not the adorablest and most violent of Darkstalkers cahracters?

Otherwise, give us a few new characters and anything that isn't R. Mika.