For villains in an X Men franchise, you have to have Stryker but also I would go with En Sabah Nur/Apocalypse and Mr Sinister for a trilogy of films
William Stryker I would go with Eric Roberts or Robert Patrick
For Apocalypse I would go with The Rock, Dwayne Johnson he's my pick for Apocalypse in the MCU. But other picks I could go for would be Oded Fehr or Rami Malek.
Mr Sinister...Jason Isaacs all day
I agree with Jason Isaacs for Sinister or maybe Mads Mikkelsen. I'm currently re-watching the original X-Men cartoon series and I swear Sinister's appearance was more (no pun intended) sinister in the original series with the pointy teeth than he was in X-Men 97.
Oded Fehr would've been a good pick for Apocalypse, though he might be too old for the role now unless they did Apocalypse as a CGI motion capture character. The Rock has played too many comedic action roles that I don't know whether he could be taken seriously as Apocalypse. If they are going for an actor to physically portray the role rather than motion capture then they would need someone with The Rock's build but obviously more serious than The Rock( The Rock's version of Apocalypse would never be able to utter the words "you're welcome" without people making Moana jokes and memes). They need someone with a commanding presence.
For Sabretooth, I'd go with Alexander Skarsgard. I quite liked the Liev Schreiber version of Sabretooth over the Tyler Mane one, so it would be good to find a balance between the two characterisations.
For Stryker, I'd go with either David Morse, Dean Norris or maybe Kiefer Sutherland.
Depending on how they characterised Mystique, Gemma Arterton might be a good fit for the role.
For Magneto, I'd choose either Timothy Dalton or Charles Dance. However, given the timeline issue with Magneto, they could have Polaris as the villain instead so they can have Magneto's powerset without necessarily having Magneto (plus they could have him pass on his ideology to her). For Polaris, I would have Amanda Seyfried.