What I've learned/heard from a few different people*:
Kollector is a real character. Friend was apprehensive about saying much more than that but he did let one thing slip that I don't wanna just flippantly throw out there (he's a cool guy and I respect him enough not to). I'll just say he's somewhat of a twist on a certain *type* of character that debuted in MK3.
CET (full name unknown, ends in N?) is a brand new female monsterish (non human) character. Through my own dumbass investigating and acknowledgement that Boon and co love taking mythologies of different cultures, I believe her name and possible visual style has something to do with Ceto of Greek Mythology. Considering Geras and Kronika are both names based in GM it makes sense to me. Release the Kraken!!(
https://en.m.wikipedia.org/wiki/Ceto)
Frost has been completely revamped. What that means I don't know.
Noob is in and sauce's sauce is pretty positive that he is base roster and not DLC, and that NRS are incredibly proud of his new design. Hype.
ST is in but they're going to keep him under wraps until launch because his inclusion is supposed to be a big fucking deal.
No other info on Rain but don't give up hope. Even with steamdb leak there's still a few slots.
No mentions of Smoke.
Base roster/DLC supposedly isn't finalized yet but I don't believe that personally. Waaay too close to release for it not to be IMO.
Bear in mind this could ALL be horseshit but don't give up hope on whatever character you want to be in.
*I have not personally seen deets. This is all from either someone relaying what THEIR source has told them, or even more removed and from a source of a source of a source lmfao. Don't shoot the messenger!