There has been lots of discussion about what to do with the next Avengers movie now that Jonathan Majors' Kang is out of the picture. They could recast and call it a variant, but Marvel seems to want to move on. Many have said to use Doctor Doom, but he should be with the Fantastic Four. I'd love to see Nightmare finally make a live action appearance, he certainly has enough power to become a franchise wide threat. Plus, Ironheart, which was to debut Mephisto, has been shelved indefinitely, so there's an opening for a mystical adversary.