I'm sure it's already been stated with FF and Rival Schools characters, but they're not the same standard of guest people are talking about. That said, I'm in the "No" camp anyway, generally.
Capcom prove they still bring out fresh, fun characters. Honestly, I'm having a blast with Lucia of all characters. And just like Lucia as the example again, they have a whole host of characters within SF/FF/RS settings that they have yet to tap into + all the existing characters having friends/acquaintances that would make dope additions e.g. some of the other dolls would be DOPE but they went with Decapre, even if some have brief appearances in Story mode. Or Azam. Or Sakura's Friend #106. Or Mel Masters, Datta, etc etc.
SF doesn't need the guests from external IP. Save those for the VS games.
That said, if it was a nostalgia tea or something, I wouldn't say no to Terry, Kyo or another SNK dude. But that's my limit, personally. And even that would just make be beg for a CvS3 instead!
I guess what I like about SF and KOF is they both tap into their own linked, mostly in-universe franchises e.g. SF+Final Fight+Rival Schools, and KOF+Fatal Fury+AOF+others. So much they already have, and plenty of great original stuff they still pump out. Confidence in their own talent, basically. No need to be like namco and SoulCalibur - guest after guest, and yet Hwang is sitting in a field alone. Psh!