3) Scarlet Witch
Here’s a left-field idea: how about the X-Men movies introduce their own version of Scarlet Witch? Due to the unique position of Quicksilver and Scarlet Witch as members of both the X-Men and the Avengers, both Marvel Studios and Fox have access to these characters. We’ve seen two Quicksilvers already, played by Aaron Taylor-Johnson in the MCU and Evan Peters in the X-Men universe, but there hasn’t yet been another Wanda Maximoff.
Peter Maximoff’s little sister was briefly glimpsed in Days of Future Past, though, confirming that she exists at least. Seeing as events have moved on twenty years from the events of that film in Dark Phoenix, Wanda would now be a young woman presumably in full possession of her powers. If she did show, Scarlet Witch would be hugely useful in a battle against Jean, as Wanda is a pretty powerful being in her own right.
It has to be said that Elizabeth Olsen’s MCU iteration is pretty far from the comic books portrayal, and this is something that an X-Men version could put right. And with the franchise’s newfound love for authentic costumes, we could even get a Scarlet Witch with her red headress from the comics.