I would love to see Kenshi, Shujinko, and Triborg.
Kenshi because he's Kenshi.
Shujinko because NRS can make him have a lot of moves from multiple characters, even ones that won't be in the game, or even moves that characters abandoned, he has a lot of potential with custom variation system.
Triborg can be a customizable cyborg with weapons and moves from Sector, Cyrax, Smoke, and even Cyber Sub-Zero, and he'll only take one slot.