Original trilogy cast X-Men:
Sir Patrick Stewart - he didn't deny/confirm, appeared in Multiverse of Madness
James Marsden - rumoured, he didn't deny/confirm
Halle Berry - rumoured, has a 2023 Ig post with white short hair
Famke Janssen - rumoured, denied
Anna Paquin - no word/rumour
Shawn Ashmore - no word/rumour
Daniel Cudmore - no word/rumour, there's another Colossus in the film
Alan Cumming - no word/rumour
Kelsey Grammer - no word/rumour, appeared in The Marvels
Elliot Page - no word/rumour
Original trilogy villains
Sir Ian McKellen - said he and Sir Patrick have "plans", could be a hint?
Tyler Mane - paparazzi photo
Ray Park - paparazzi photo
Rebecca Romijn - rumoured
Bruce Davison - no word/rumour
Aaron Stanford - seen in trailer 1
Kelly Hu - seen in trailer 2, if she was the one playing Deathstrike again
Brian Cox - rumoured
Vinnie Jones - declined the offer
Dania Ramirez - seen in trailer 2?, if she was standing next to Toad in the group shot
Others
Jason Flemyng - seen in trailer 2, if he was the one playing Azazel again
Channing Tatum - rumoured
Taylor Swift as Dazzler - rumoured
I'm digging some rumours for this list. Its crazy how Deathstrike, Pyro and Deathstrike weren't rumoured before.