Jason Issacs as Magneto
Magneto should play a major role with the Mutants coming to the MCU before getting to the team Professor X built. Who they face is equally as important. The Scarlet Witch could be responsible for leading the Mutants into the MCU. She could introduce Magneto, Mastermind, Toad, and a new Quicksilver as the first mutants in the MCU. Their tactics would force Professor X to form a group of his own.
Being a Holocaust survivor of Auschwitz means everything to the essence of who Max Eisenhardt is — don’t change that. Given it all happened in the 1940s, Magneto would be over 100 years old — that’s okay. The MCU can explain how the X-gene allows mutants to age slower as they get older. Thus Magneto looks 60-ish despite being over a century. Who should play Magneto?
For a while, Liam Neeson was a fan cast favorite. He’s underrated in terms of his acting prowess. Stephen Lang was another, but he fits better as a brute like Cable (if the MCU recasts). A name no one is talking about (except for the Random Rants Podcast) is Javier Bardem or Viggo Mortensen, who has the silver fox look.
Jason Issacs, however, would nail the role of Magneto. He has the commanding presence and look you want for the character. More importantly he has some underrated acting skills. He’s mostly known for his role as Lucius Malfoy the commander one of Harry Potter’s big bads. From a cultural perspective he’s Jewish, which is a plus. He recently played a lieutenant commander in the British war drama Operation Mincemeat. This is almost too perfect a fan cast.