I think that, similar to their strategy with Spider-Man, Feige and friends will try their best to avoid using the same characters that were used in previous films. At least early on. The good thing about the X-Men is that there are SO MANY different characters throughout its history that you can assemble so many different permutations of the team
That said, I wouldn't expect to see Mystique in the first couple of movies, as her character has gotten more than her fair share of attention already. Same goes with Dr. Doom. I think they will save him until the groundwork has been properly set up because A) he is a major villain who can be the next Loki, crossing over between solo F4 and Avengers films and B) the character has been besmirched by bad appearances in all 3 of the recent Fox F4 films. they will not risk screwing him up again. Magneto, I think, will only play small part, if any, in the first movie.
The MCU likes to focus on the heroes first, so I think we could get a group of minor X-Men villains or a generic alien race for the first film (like the Phalanx, which could set up Magus as a villain in future films). Or at least a never-before-seen major villain like Mr. Sinister (and the Marauders) or Mastermind. Possibly Omega Red
As for casting, that is a long time away. Supposedly Feige and co. were not allowed (legally) to do ANY work on X-Men until the Fox deal was finalized. Which means that they are just BARELY starting work on a new story, which characters to start with, character designs, how to tie them in to the greater MCU, etc.
Which means that they probably won't start casting for at least another 3 years.
For Prof X and Magneto expect to see seasoned actors with a great pedigree, quite likely with Academy Award nominations or wins. Likely from the UK or Australia or Eastern Europe. Probably in age ranging anywhere from mid 40 to early 60s. Young enough to do these movies for at least another 10 years. Magneto, especially, should be young enough to do some action sequences. Mark Rylance would make a great Prof X. A guy like Ralph Fiennes could probably do either role. Possibly Gary Oldman, but not likely IMO
For Cyclops I'd expect a generic white male leading-man type (LOL). Guys like Billy Magnussen, Jack Reynor, Jack O'Connell. It depends on what age group they decide the original X-Men team will be. These actors are in their 30s, but Marvel could decide to go younger, like early to mid 20s
Also keep in mind that Marvel has no problem changing the race of characters, like Nick Fury, Ancient One, Mary Jane, etc. so you cannot just look at the usual suspects. Actors of color are likely to be cast in some of these roles
The MCU doesn't cast unknowns in the main hero roles, but it also does not cast huge names. Actors like Chris Evans, Jeremy Renner, Elizabeth Olsen, Aaron Taylor-Johnson, Chadwick Boseman, Chris Pratt, Anthony Mackie, Brie Larson, etc. were not huge names, but they were established in the industry. Robert Downey Jr., Paul Rudd and Benedict Cumberbatch had more starpower, but not by a lot. They generally hire long-time veterans only for character roles, like Tommy Lee Jones, Samuel L. Jackson, Anthony Hopkins, William Hurt, Robert Redford, etc. characters that are one-offs or only do cameos throughout the series
I would love to see Rosario Dawson as Storm but mybe she's too old? Also she was on Daredevil show , so maybe no , Zac Efron as Human Torch , Charlize Theron as Sue ? Maybe Christian Bale as Magneto ? What about Scott Eastwood or Jason Momoa for Wolverine? Other actors i would love to see in MCU are Viggo Mortensen , and like you said Sean Bean
She'd make a great Storm, but yeah, she's probably in her 40s now? Still as gorgeous as any woman out there, but I think they'll go for someone younger. Maybe Samira Wiley, Kat Graham, Sonequa Martin-Green, or Keke Palmer
The auditions for the supporting X-Men characters will likely be massive. Every young actor in Hollywood will be going for one of the X-Men roles just like they auditioned thousands for Spider-Man homecoming
Wolverine will be one of the hardest parts to cast. Jackman was good, but lots of people pointed out how he doesn't look like the comics Wolverine. Logan is supposed to be very short, 5'3", and burly. Jackman is 6'3" and looks like a male model. So I think Marvel will end up hiring someone under 6 feet tall
And yeah, Viggo and Sean Bean are great in everything they do. Viggo would be a perfect Magneto, but I think he is the type of actor that prefers not to take on huge commitments, so maybe he could play Mr. Sinister
I'd also like to see Zac Efron take on a comic book role. He obviously has the right look, and I've seen lots of potential in him since those cheesy Neighbor comedies. He brought something more than just a goofy idiot to that role, IMO
Since Mr. Sinister (Nathaniel Essex) is British in the comics, and Marvel (and Hollywood in general) loves to cast Brits as villains, you can expect one of the many talented british actors out there to be cast in the role
Guys like Colin Firth, Ewan McGregor, Sean Bean, Clive Owen, Hugh Laurie, etc. they could cast just about anybody short of Daniel Day-Lewis. Now that the MCU is such a huge phenomenon even big A-listers are willing to sign on for one-time appearances