I mentioned it yesterday... I think the "big bad" will be just a "god" version of another character, maybe even Reiko. Or we could even be in for a new Deadly Alliance of Shinnok + "god" character. Onaga would be cool but each day I'm more and more under the impression that it has nothing to do with him, just a literal BIG version of a known character.
As for costumes, you're all reading too much into it. If it's supposed to be a surprise, that could well be part of the plan to surprise us. Plus, if it's indeed a "god" version of one of these characters, they could change in their big bad version. I also can see Reiko as a possible sub-boss, in all honesty (if Havik turns into a god). Another thing to consider is that IF they really are trying to surprise us, they could be hiding some of these characters and that's why we haven't heard about them being in the game yet. It would make sense, actually: if Reiko, Havik, etc are unplayable bosses, they would try to keep them a secret so we would be trying to discover if they are in or not. That would be better than just throwing out in the wind: "this is the boss!!!"
PS.: Or Noob's ending was canon, he freed Havik, Havik's his pal and the big bad is some sort of god Noob Saibot.