I don't mean characters who come from another dimension, like the DC characters in the crossover. With other dimensions/alternate realities, any characters from any series could crossover with MK. Even the looney tunes. No, what I'm talking about, are non-MK characters that could plausibly exist within the MK universe itself, without contradicting any core aspects of MK's story, and without needing much tweaking.

Who could exist in the MK universe (imo)

Spider-Man: His origin doesn't contradict anything about MK's story.It has nothing to do with gods, and the supernatural (at least the mainstream origin doesn't). He could plausibly have been involved in the fight against Shao Kahn's forces when they invaded Earth. Many of his foes could plausibly exist in the MK universe too, like Doc Ock, the Green Goblin, Venom and Carnage. Just Spider-Man and some of his rogues' gallery, not the entire Marvel universe.

The Alien and the Predators (from the appropriately named franchises): So easy to fit in. The Predators might be able to reach all the realms from space (if they are planets). If not, they could certainly use portals. As for the Aliens, they could be on any planets/realms not yet discovered by mankind. Given that there's still so little we know about the Aliens' origins, they could have been created through magic, if they were a part of the MK universe. There's a lot of other extraterrestrial/non-human beings from other series that could fit in MK, like the Thing, Sil (Species), the Critters, the Blob, the apes from the "Rises of the planet of the apes" remake, and of course the Engineers from Prometheus, which is related to the Alien franchise.


Who couldn't exist in the MK universe

Kratos: and the entirety of his world. The gods there are directly involved in the lives of human, while the elder gods avoid direct intervention as often as possible so not to draw the One Being's attention. Where would the Greek gods fit in MK's mythology, and why wouldn't the One Being be awakened by their presence? Successfully merging MK and GOW would take a great deal of effort, not the kind that likely would be made merely for a crossover of two series belonging to different companies, or guest appearances/cameos.

X-Men: The organization itself, not necessarily some of the individual characters. MK is filled with superhumans, and they don't seem to suffer from discrimination, from what we know so far. One of them is even a movie star, and two of them work for the military. So the X-Men organization's existence doesn't seem to be much of a necessity. I could see the more inhuman-looking X-Men like Nightcrawler being discriminated against, but the likes of Storm, Cyclops and Charles Xavier himself? Probably not in the MK universe.

Spawn: Heaven, Hell and the supernatural in general are fundamentally involved in his story, and they're all significantly different from MK's counterparts. Hell alone largely differs from MK's netherrealm, with it's seven spheres. Crossovers and guest appearances pretty much requires making the two universes alternate dimensions/realities, like the DC crossover, in order to work. Still, a few Spawn characters could plausibly exist in the MK universe, like Sam, Twitch, and Overtkill.