6) Scarlet Witch
Scarlet Witch is obviously a prominent part of the Marvel Cinematic Universe (most recently seen in Captain America: Civil War), but she can still appear in the X-Men franchise. That’s possible because she’s one of only a handful of characters who both Marvel and Fox can use due to her comic roots being equally weighted in both the Avengers and X-Men books.
X-Men: Apocalypse finally confirmed that Magneto is Quicksilver’s father – a plot point Marvel is unable to take advantage of – so having both him and his sister appear as part of the Master of Magnetism’s Brotherhood of Evil Mutants could be a great place to pick up with the villain when we next see him. Sure, Peter had decided to join the X-Men at the end of Apocalypse, but a lot can happen in a decade, and it’s fair to say the vast majority of fans would be pleasantly surprised to see this brother and sister team now standing alongside their father rather than Professor X.