In a recent interview with Mad Catz, Mortal Kombat X Creative Director Ed Boon spoke about the possibility of guest characters making an appearance in the game as DLC.
"That [guest characters] was something that was embraced in Injustice [Gods Among Us] and in Mortal Kombat," Boon said, noting that Freddy Krueger and Scorpion were the most-downloaded characters in Mortal Kombat and Injustice: Gods Among Us respectively. "I think there's a novelty to having a downloaded [sic] guest character, so, I think the chances are pretty high. I don't think every single one of them [new characters yet to be announced] will be a guest, but I think the chances of at least one of them will be pretty good."
Can't wait for all of the "Waaaah" MK purist threads