Fighting game guest characters are a staple within the genre, and one of the best to ever do it is NetherRealm Studios. The developer behind Mortal Kombat and the Injustice series has produced fun, wacky, and memorable guest characters ranging from The Predator, Hellboy, The Teenage Mutant Ninja Turtles, and Spawn. With Mortal Kombat 12 rumored to be in development, it's anyone's guess as to who will make it on the roster.
Mortal Kombat has already included '80s action heroes, a slew of horror icons, and DC's Joker as guest characters in previous games. Assuming that Mortal Kombat 12 is in development at NetherRealm Studios, coming up with new characters that haven't appeared in previous games might be difficult. But going off of NRS' past inclusions, there are a few plausible guest characters that would be surprising, but also help add variety to the game's roster.
John Wick
While it was previously stated that NetherRealm Studios wanted to get Neo in Injustice, perhaps a more fitting option would be to go with Keanu Reeves' more recent action hero, John Wick. What would make John Wick an interesting fighting game character is how he might play. While the obvious route for John Wick would be to have him use his gun and be a "zoner" character, it'd be interesting if NRS decides to turn him into a "command-grab" character. The character in the movies is often seen getting up close and personal, performing various grabs and takedowns. John Wick could go far beyond just shooting a gun. It would also be cool to see him whip out a pencil and use it in one of his fatalities.
John Constantine
It's more than likely that a future Mortal Kombat game will also include a guest DC character that crosses over from Injustice. There are plenty of established Injustice characters for NRS to choose from, but a shocking, yet plausible reveal would be John Constantine. The demon-hunter has a strong, underground following with appearances in DC's animated movies, live-action TV shows, and a 2005 live-action film starring Keanu Reeves. Although, he's very much overdue for a large video game role. In terms of play style, Constantine could fill the fighting game archetype of a "summon" character, conjuring up demons to fight for him. This would also help fill an archetypal void within the Mortal Kombat roster.
Doomguy
It might be difficult to include Doomguy now that he is a Bethesda/Xbox character, but it's not impossible considering Mortal Kombat (2011) featured Playstation-exclusive Kratos. Doomguy is such a natural fit for the gory and bombastic style of Mortal Kombat that it's hard not to picture him in a future game. While Doomguy has many iconic weapons like the double-barreled shotgun and BFG, he also has the potential for mobility, utilizing his dash and grapple hook to get in close and deal some heavy hits.
Leon Kennedy
While Ryu from Street Fighter will not be making an appearance in Mortal Kombat anytime soon, Leon Kennedy from Resident Evil certainly can. In the past, Capcom hasn't allowed Ryu to crossover into Mortal Kombat because Capcom doesn't want to show Ryu being decapitated. With Leon, however, the same restriction doesn't apply. In the Resident Evil games, he's seen many times being brutally murdered and "murdering" zombies. So there's a chance Capcom might be okay with him performing fatalities and having other characters perform fatalities on him.
To get an idea of how Leon might play in-game, fans can see how Chris Redfield plays in Marvel vs. Capcom 3 or look at Jill Valentine in Marvel vs. Capcom 2. Leon Kennedy will be heavily featured in Resident Evil 4: Remake coming soon, so it wouldn't be surprising if NRS capitalizes on his popularity, while also fulfilling fans' long desire for a crossover.
Mortal Kombat 12 is rumored to be in development.