You’re already entering weird territory by having the third Deadpool movie suddenly happening within MCU continuity, so why not Wade Wilson’s fourth wall-breaking nature to pull in those in his inner circle?
Ok, maybe that might be a little more complicated with Josh Brolin’s Cable since the actor already played Thanos, but as far as Beetz’s Domino is concerned, it’s doubtful that would be an issue.
Considering that Zazie Beetz hasn’t appeared in the MCU yet, not to mention how her stardom has grown following Deadpool 2’s release through movies like Joker, Nine Days and Lucy in the Sky, it would certainly be easy enough to put her Domino back into play rather than recast the role.