Jason would be awesome (I’m a huge Friday the 13th fan – Kane Hodder best Jason), but that would instantly kill their own game. It’s a weird franchise when it comes to collaborations though. Slayaway Camp has another game that features Jason exclusively (Derek Mears did voice work for those games I think) and he’s in Mortal Kombat X too, of course. Dead By Daylight is way too similar though...
I would love the addition of famous scream queens. Not sure if they’re already in the game (haven’t played it yet, but I want to try it out soon). Jamie Lee Curtis is in the game, I think? Or just her character from Halloween? Anyway, Linnea Quigley, Linda Blair (I guess she counts), etc.
Alien and Predator wouldn’t fit at all, in my opinion. Slashers only! Maybe some old school classics like Maniac Cop, Sleepaway Camp (Angela would be AMAZING, now that I think of it).
Edit: Oh! Oh! Evil Dead!