Professor X and Magneto: Cast the MCU version!

Thor, Loki, Warmachine, Scarlet Witch, Quicksilver, Spiderman, Black Panther, Valkyrie, Falcon, Winter Soldier, Drax, Ant-man, Yondu, Nebula... all were played by relative unknowns. Or at least by actors who are not readily recognizable to the general public. Even Chris Pratt wasn't that recognizable a name.

And that's not even including the villains.