As speculation over Mortal Kombat X's content future defiantly simmers post-MKXL; GameInformer have tapped into the always engaging, often contentious subject of guest characters - past, present and future.

Ed Boon revealed the best selling downloadable characters of the most recent game were Alien and Predator. It follows a trend started by Freddy Kreuger and Scorpion, who topped DLC sales in Mortal Kombat (2011) and Injustice: Gods Among Us respectively.

Mortal Kombat X featured the most downloadable characters of any NetherRealm Studios fighting game to date: eight in total. Four of those were guest fighters, meaning the combative Alien vs Predator mega-franchise trumped classic horror icons Jason Voorhees (Friday the 13th) and Leatherface (Texas Chainsaw Massacre) in latest sales. That fact hasn't stopped Ed Boon being open to the idea of a dedicated horror fighting game [full story].

On the appeal of the crossover characters, Boon says, "There was a coolness and a novelty factor to seeing the mixing of the things." Supported by a history of monetized success, Boon noted of guest fighters in Injustice 2: "... I would be surprised if we didn't do any of that." Potential good news for those who enjoyed Scorpion in the first DC Comics fighter, and may be holding out hope Todd McFarlane's well documented interest in a Spawn crossover.

How does a character like Spawn make it to the roster of another company's fighting game? The tangled web of corporate licensing and intellectual property has the final say, but Boon broke down the basic process for GameInformer:

"Probably the biggest one is simply someone suggests someone," he says. "With Mortal Kombat, we stepped outside of the universe with characters like Freddy Krueger, Jason, Alien, Predator, and stuff like that, so the movie stuff kind of mixes things up. Fan feedback is another big one; just hearing some very loud, insistent people saying what they want. And what's available; obviously we can't just choose what we want to do, like 'we want Robocop so he's in the game!' You have to do something that can be accommodated as far the company. So there are a number of variables, including what the players have been asking for. We're always listening to feedback."

For every guest character that makes the cut, there are several that don't go beyond consideration. In the past, Boon has revealed several fights they almost started with: Sweet Tooth (Twisted Metal), Marcus Fenix (Gears of War), Michael Meyers (Halloween), and characters from the Terminator franchise.

Comic book creator Todd McFarlane has also been notably interested in his independent creation Spawn crossing over with both Mortal Kombat X and Injustice. He has claimed to have Warner Brothers permission to use the character in the past, even though nothing came of it.

Mortal Kombat characters themselves are no strangers to being invading presences. Characters have been crossing over with klassic Midway arcade games since NBA Jam. Their in-house proliferation has continued with Kombatants in titles like The Grid, Injustice: Gods Among Us and WWE Immortals! Appearanes in Unreal Championship 2 and NFL Blitz made Raiden one or Mortal Kombat Online's Top 10 unlockable kombatants!

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