Boon stated that we are to get a "fun" and "surprising" returning character for this... And I highly doubt anyone knows his definition of those words I quoted.
I don't see it being Liu Kang, not because it's predictable, but I just don't see it at all. He's literally a character I do not see at all being a threat to take out, even if his arcade ending is to become true and he is this fire god many want to see him return as. In my opinion, I think that's a dumb route to take. I don't want him being this character that has to go through everything. A human, a zombie, undead, and now a god? Come on.
Honestly, I see it being Shang Tsung. He's not technically dead, as stated in a Tweet and such, only his "powers" were transferred to Sindel. So that basically means he's still out there, either living, or he's in someone's possession. And I really don't think Quan Chi is in possession of him at all.
I don't usually theorize as much as a lot of fans do, but I'll throw out this. I see Shinnok sitting in the Netherrealm ordering those slaves around while Quan Chi is roaming around the realms doing their plan and such. Shang Tsung is among the slaves that Shinnok has. if there's a Netherealm War and such, I can see him somehow coming back to life by fighting against the Netherrealm. *Insert mysterious way he gets his powers back* He leaves the Netherealm and is out looking for Quan Chi *Insert random death of Quan Chi killed by whoever* and then it's Shang Tsung who rises from above. Because I hardly doubt Kotal Kahn is going to be that much of a threat, hell, maybe he and Shang Tsung cross paths and Shang gets him.
Blah, really, I see Shang Tsung being the end boss out of all the characters many think will be. I really, really don't see it being Liu Kang. I think, really, if he does, that will be the only thing I'll ever be disappointed about X. I look forward to the final battle/end bosses, and some games I've been really disappointed with the end bosses being lame and not that fun to fight against. I'm hoping that they think twice about it being Liu Kang.