Stryker. He's a cop, and it could have been a shooting game.

ya probrbly scorpion

I think it would be cool to see Shao Kahn for ALL of his conquests, including when he invaded Edenia, Zeterra (SP) and the Vampire realm. But it would be cooler as a book, not a game.
Scorpion would be cool, Maybe Baraka.

Tough call here. Almost every character could have a spin off MK game based on them. I love the idea someone stated about the Stryker/Doom style shooter game. It would be different. We've all seen ones about good guys, but not too many on bad guys. Perhaps a game on Reptile would be interesting too. We know very little of Reptile's past, and like Mythologies it could explain a little. All we know is that Kahn destroyed his realm. Who was Reptile exactly in his realm? Guard? King? Citizen? One other possibility is maybe a follow up game for the younger Sub-Zero where we learn more about him, and the cyborgs.