Well, for one, he would make sense in the chronological sense of the story. He comes in around the Deception portion which, if you saw a previous timeline post of mine, would be the story after MK4 in the traditional sense since DA wouldn't be around (no more shang). Since we are on a separate timeline, however, a lot of characters are either dead or wouldn't partake (Kai) due to obvious reasons of necessary connections no longer happening (namely Liu Kang being gone now). Onaga doesn't per se suffer from this problem, since the only real ties he has are based around Raiden (likely to return), Quan Chi and Shinnok (who should still be around) as well as their medallions. Reptile would need to exist as a catalyst if they reuse that timeline point, which since we never see him die its fully possible.
As it stands: Quan Chi discovered his tomb, so theres no real reason that he
couldn't be in the game. All of the factors required to justify his existence and add him are still around. Whats more is he is a huge plot point in Deception, and with Shao gone (like in DA where he was overthrown by Quan + Shang), the grounds are remade. Netherrealm wants conquest, they'll need an army. What better than someone whose power rivals The One Being who fought the Elder Gods so long ago, capable of resurrection and already holding a vast army under his command? If MKX turns out to cover the area of the story that MK: Deception would have done (considering the 25 year span, with the story covering parts outwards to the end time, its somewhat likely) then perhaps a power move to really turn the odds against Raiden and his crew.
His styles would complement it, between perhaps a fire centric style, one utilizing his slams and powerbombs, and another based more around his ability to fly and do jumping stomp strikes. His fatalities are a fresh bit of ground, since he doesn't have any of his own as of yet. Could be a nice place to flex that creative muscle.
Anyways my take on it, all this aside? I wouldn't mind seeing him, perhaps him mistaking Kotal Khan for Shao would be ample enough reason to get in on the action. That's one mighty big throne, someone has to fill it. I don't know if he is the reveal (I don't think it will be raiden since he hasn't been waiting for an eternity, but Fuijin would be hype), but all things considered there isn't any reason I can think of that says he
isn't the reveal as well.
For those who are newer to the franchise and don't know who Onaga is.
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