If they want to keep introducing new characters for future Mortal Kombat films, they're already starting to run a little low on not-too-silly characters.
They've established this Mortal Kombat universe as rather realistic for Earthrealm and more of a fantasy setting for the tournament, so I'm not sure they'll want to introduce all of the cyborg-ninja characters, so that removes a bunch of possible ones already.
The ninja recolors like Rain and Smoke also probably not going to work out. They can do Ermac because the recent games have changed his design and powers enough that he's his own thing now, and he's quite badass with cool visual abilities so I think he'd be a good pick.
Reptile also could work, since he's not really a human character they can go in the direction of him having a reptilian face and claws and spitting acidic venom, anything would be better than the CGI abomination of 1995 Reptile.
Nighthawk obviously isn't gonna be a thing in this day and age, Stryker probably not for similar politically correct reasons. Kotal Khan also probably not gonna work out in case you offend 3 Aztecs somewhere.
I'd like Kenshi to be introduced, he has cool abilities and it would make for some great fight scenes I think (blind guy who can control his katana with his mind, it flies around).
Erron Black is a little silly as a concept (basically a cowboy mercenary in Outworld), but he's pretty badass. However, I think guns would kind of go against the current vision they have for the tournament, we've only seen blunt or bladed weapons.
Kintaro would be too much of a Goro redo, Sheeva also plus you can't really do much with four arms for cool choregraphy I think that's not already been done for Goro. Motaro is a bit silly.
And obviously the "kids" from the recent games (Johnny and Sonya's daughter, Jax's daughter, and a few other young characters) aren't a thing in the movie timeline.
My predictions for the next film characters :
Sonya
Raiden (just mentor role, might pass the torch to Liu Kang early into the story)
Liu Kang (as Fire God perhaps)
Kung Lao (brought back to life as per Liu Kang's vow)
Sub-Zero (the nice one, Bi-han's brother)
Kano (comic relief in every film)
Johnny Cage (if they spend money on Karl Urban again but I'm not sure he'll be the main character this time around)
Jax (resurrected)
Kitana (eye candy and token girl boss)
Jade (would be cool if she wasn't bald this time around, and also pretty)
Mileena (important role if they want to do a girl boss story)
Reptile (throwaway character for one fight)
Ermac (throwaway character for one fight but memorably badass)
Shang Tsung (different actor because he becomes young again, which is his objective in several games)
Quan Chi (plot necromancer)
Shinnok (big bad)