@ Araknyd
Johnny or Sonya one will not be back now. Possibly both. They have their daughter to thank for that.lol.
Sure she doesn't play exactly like them, but she has a style which resembles each of them. Why put characters in the game when we have one who resembles both pretty similarly. Plus, Cassie is awesome!
I say either Johnny or Sonya will be there. Not both though.
As far as Sonya and Johnny are concerned, you seem to have forgotten Frost who also had similar moves to Sub-Zero, but was also in the games along with Sub-Zero.
Frost never was a replacement for Sub-Zero, just as I don't think that Cassie is a replacement for Sonya and Johnny.
Also, Ed Boon said that someone brought up fatalities (on family members) and Boon was like "Oh, shit I didn't think about family members" but that he'll throw them in anyways (regarding doing fatalities on each other) in the GainBomb interview (4:10 mark).
Boon also said that "aged" MK characters would be in MK X, and that MK X would show how they got there (PlayStation stream).
Given that MK X starts close to the end of MK X, Sonya and Johnny make the most sense, given that those 2 and Kano are the only
mortals that survived MK9.
Kano having been a flat lackey character can be ruled out for MK X, which leaves Sonya and Johnny as the only 2 surviving mortals who would actually age in 25 years.
Everyone else who survived MK9 is ageless / immortal / 500+ years old.
As for more "ninjas", that could also mean Ermac, Rain, Smoke or Reptile.