Ghirahim would've been perfect. He would've been a new Zelda representative. Have no clue why they so stingy on Zelda characters.
Sephiroth, but he would require his own moveset
Akuma, easy Echo Fighter for Ryu (alongside Ken)
Eggplant Wizard, but would require his own moveset
Giovanni, but would require his own moveset (could switch between Persian, Nidoqueen, and Nidoking like Pokemon Trainer)
Bass, could be an Echo Fighter for Mega Man
Love these suggestions especially the Giovanni one.