I personally think Sin is a must. He's so deeply ingraved in the story, if the next GG takes place after Overture, he has to be in it. Not to mention I don't think we've seen all of his abilities.
Valentine is still shrouded in mystery. It's only logical that she'd get some more light shed on her character in a new GG.
I could write a couple pages on why Raven should be in the game. But I guess I'm a bit biased The guy's character was SO well fleshed out for being in a single game. From his gestures to the things he said to every last peace of his design. They'd be crazy not to put him in.
Izuna and the Dr. are both superb characters, and I hope they make it also. Although about your no new characters statement, I also think that alongside Raven and I-No, this guy should finally be revealed:
It's about time we get a good look at That Man's third servant. Hopefully they'll pimp him with a badass new design though, so he doesn't look as much like a KKK member and show his face :/