[Guest characters] was something that was embraced in Injustice [Gods Among Us] and in Mortal Kombat. I think there’s a novelty to having a downloaded guest character, so I think the chances are pretty high. I don’t think every single one of [the 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.
As you’ll recall, Kratos was a playable character in the PS3 version of Mortal Kombat, while Freddy Krueger was a downloadable fighter in Mortal Kombat and Scorpion was a downloadable fighter in Injustice: Gods Among Us. Boon noted in the interview how Freddy Krueger and Scorpion were the most-downloaded characters in their respective games.
Mortal Kombat X will be released in 2015 for the PlayStation 4, PlayStation 3, Xbox One, Xbox 360, and PC.