Here's who I can imagine being in the next Mortal Kombat:

(From vanilla MK9)
-Johnny Cage
-Sonya Blade
-Raiden
-Lui Kang (Rivalry with Raiden, over the deaths that said person caused)
-Cyrax
-Sektor
-Reptile
-Ermac (Now reformed, following Shao Kahn's death and becomes King Jerod. He wants to free his wife and daughter's souls from Quan Chi)
-Scorpion
-Quan Chi (Hunted by Scorpion)

(For MK10)

-Rain
-Reiko (him and the former being in rivalry over Shao Kahn's throne)
-Hotaru
-Havik (him and the former being in rivalry over the anarchy in Outworld)
-Kenshi (allies with the reformed Ermac and helps him release the dead MK9 characters)
-Skarlet
-Fujin
-Shinnok

-Two or three new characters (possibly DLC)