I'd like Brandon Lee as Sub-zero, but I will agree that he'd work better as the younger brother that replaces the old one... or you could have him be the old one and have him stay on if they create Noob Saibot as a nod to the Crow as a person who came back from the dead. Either that or Scorpion as mentioned, since he's proven he can do something like him, an avenging soul come back to seek revenge on the Lin Kuei for wiping out his family. If he's busy though, Sub-zero II for the sequel it is. Hell, you could arguably do a decent spin-off with that, since there's the whole "roboticize the ninja" plan that the Lin Kuei had.
As for Goro; for some reason my brain is going Kane Hodder for his big fuck off screen presence, barring some prosthesis or something, with a voice over or not it's up to you. He can do pretty well in a role as an unstoppable monster.
Reptile, you'd need someone who's wiry and fairly acrobatic IMO. He can be a minor presence for the first, being an agent of Shao Kahn out to watch his sorcerer.
