That’s been seemingly confirmed from the pictures and footage showing Professor X, Cyclops, Nightcrawler and Storm at a funeral (I suspect it will be Mystique biting the dust).
As for familiar faces, most of the players from X-Men: Apocalypse will be back, from Professor X and Magneto to Quicksilver and Psylocke.
That said, it was rumored last year that one of the characters in Dark Phoenix who’s been around the franchise since 2011’s X-Men: First Class, will be killed off.
So in addition to the standard civilian casualties you come to expect from superhero movies, it looks like someone important will be departing to the great beyond during Jean Grey’s rampage.