I'm sure this conversation has come and gone already, but I think the reason why we're all suddenly so fixated on characters like Chun-Li and Robotnik/Eggman (aside from "leaks" and lucky guesses) is because Sephiroth's reveal pointed a giant spotlight on other third-party fighters that feel exceptionally missing in hindsight.
Chun-Li is one of gaming's biggest women both in terms of influence and popularity. She's arguably the second most iconic character in SF and almost always the second SF rep included in crossovers. Seeing Ryu and Ken get in without her doesn't feel right.
Seeing Sephiroth standing next to Bowser, Ganondorf, Dedede, Wolf, Ridley, and K. Rool makes Robotnik's absence feel all the more noticeable. He's literally the last truly iconic gaming villain not to be here.
I'm sure we'll be seeing a lot more "leaks" pop up in this Post-Geno Era filled with characters from already-represented games.
I think that's the irony of the whole "Everyone is Here" promise. With such an insane amount of characters getting included, we actually start to look at the roster and feel like certain characters have been excluded.
Bizarre how our brains work.