if any character was a mewtwo reveal, I would assume they would be pretty last minute. I think people forgot the sheer chaos that was caused by no mewtwo in smash 4, even before he wasn't confirmed to be in launch people were ****ing going ape over seeing lucario in smash 4. That announcent was probably a last minute thing to make people chill the hell out.
I know verge said square enix yada yada, but if Sakurai saw a character not be announced, go "oh god" and decide to do a similar last minute addtion in dlc, wouldn't that be Skull Kid or even Waluigi, the Waluigi garbage hit like national news and forced a response from reggie, I would be surprised if the last minute DLC request character would be Geno over those 2.
I mean, the SE character could be DLC and like, erdrick still, but Mewtwo seemed to be a special case in response to the community going buck wild, i imagine if we get a pre release DLC announcement, it would be a similar case.