Since Samurai Goroh and Dr. Stewart are both trophies, the only other "original four" F-Zeor char if I remember irght, is Pico. And I doubt anyone likes him.
The most likely X Candidate would be Black Shadow. GX? Well, Deathborn, probably, but I'd prefer Princia.
They probably won't bother with the Maximum Velocity or GP Legend characters, although there is a chance they'll make a reference to the anime continuity with it's "Neo Captain Falcon".