Magneto- Charles Dance
The biggest problem with getting Magneto into the MCU is that his origin is based on a real world event (The Holocaust) and the MCU is currently in the year 2023. Even if you put Magneto born at the very end of World War 2, you're still looking at an aging character in his late 70s. Add to the fact that mutants aren't widespread in the MCU (I think the snap will be used as the reason why mutants are suddenly so widespread when they weren't before) and you have a difficult situation of trying to stay true to the character's ideology when it comes to mutants vs humans yet there's not been that conflict before now.
I envision MCU Magneto to have been born at the end of The Holocaust so his experience of that terrible event is through the impact of the long-lasting damage it had on his family. This would currently put him at about 78 in the MCU. Maybe he could be the owner of a WW2 museum so he's well versed in the history of that time and the growing anti-mutant sentiment begins to display some alarming similarities to that history which is what plays into Magneto's pro-mutant/anti-human stance. I would actually kill Magneto off after one movie for three reasons: 1. Because of the character's impending age- there will come a point when it looks ridiculous that the X-Men struggle to defeat an elderly man, 2. The character has already been a main part of both the original X-Men films and the prequel films over the last 20 years so it would be a change to focus on different characters rather than playing the same dynamic over and over again, and finally 3. Magneto's daughter has more or less the same powers as him so any scene that requires magnetic manipulation of metal can be done by her.
At 74, Charles Dance is within the ballpark of the character's age plus he has a commanding presence on screen which would fit Magneto's personality.
Professor X- David Suchet
Patrick Stewart did such a good job as Xavier in the previous X-Men films, it was really hard trying to find someone who could take on the role convincingly, I think David Suchet could do it. Whilst Xavier's age isn't constrained by a real world event, unlike Magneto, I think they should be around the same age to really sell that old friends vibe and fortunately both Suchet and Dance are the same age. Likewise with Magneto, Xavier has been a main character in the previous X-Men films for the past 20 years and really he's only required to give the X-Men their team name in the MCU. Therefore, I'd also kill his character off relatively early, the X-Men can call themselves X-Men to commemorate his memory whilst Jean Grey can take on the telepath role in the franchise.
I envision MCU Xavier to perhaps be Jean Grey's biological father (she took her mother's name since they were unmarried) so for both Xavier and Magneto, there's the element of their respective daughters carrying on their mantles. Since Cyclops would be married to Jean, it would also give a father/son dynamic to the Cyclops and Xavier relationship which would explain Cyclops' loyalty to Xavier's beliefs and methods.
Lila Cheney- Freema Agyeman
In some versions of the comics, Lila Cheney was a liaison who basically set up jobs between Gambit's group of thieves and people wanting to hire them so I would use that aspect in the MCU which would tie Lila's character to Gambit and Storm. Since the character hasn't been featured on-screen before, I think there is opportunity to play around with aspects of her character whereas other more well known ones are kind of set in stone.
I envision MCU Lila Cheney to own her own bar/club where she also occasionally sings (since in the comics she was a former rock star) but she tends to hire bands so she can actually focus on running the bar/club. Her liaison business is a side line that brings in some extra cash since she gets a cut of the profits for actually setting up the transaction. She'd be another morally dubious character who is useful to the heroes.
I think Agyeman would be good in the role as she gives off the vibes of someone who takes no nonsense from anybody and is not afraid to stand up fpr herself.
Polaris- Lily James
Polaris has recently been onscreen in The Gifted which I think is the character's first live action appearance so she's kind of an up and coming character compared to the more well established ones.
I envision MCU Polaris to initially be kind of naive (like she was in her Wolverine and the X-Men portrayal), always trying to see the good in people etc, but eventually as time goes on and anti-mutant sentiment grows, she begins to lose that innocence as well as be influenced by the attitude of her father. After her father's death she hardens up and vows to continue on his work fuelled by her grief which creates an anger towards humans. I think whilst MCU Polaris would ultimately like for everybody just to get along, she considers it a naive idealism that is never going to happen and there is kind of this vicious cycle where everyone is on the defensive against each other so things just keep escalating. Eventually Polaris would join the X-Men which would allow other villains to take centre stage so the Xavier/Magneto dynamic wouldn't play out too long.
I think Lily James would be good for the role because she can play happy go lucky idealism quite well but I also think she could convey the right emotion and vibe as Polaris' idealism starts to be stripped away and she becomes more cynical.
I wouldn't want the Polaris/Jean Grey dynamic to be seen as basically a gender bended Magneto and Xavier dynamic hence why their opposition to each other wouldn't carry on too long.