Cassandra Nova

Like Madelyne Pryor on this list before her, Cassandra Nova would have to appear in an X-Men movie, due to her associations in the comics. She is neither a mutant nor a human "“ and she's also completely intangible – and would represent a very different kind of threat to anything seen in the franchise to date.

A member of the parasitic species of beings known as the Mummudrai, Cassandra Nova was born bodiless on the astral plane – having essentially been created by the negative psionic energies on the physical plane. They are essentially dark reflections of humanity.

Every now and again, the Mummudrai crossover to the physical plane "“ something Cassandra Nova has done several times – and, given that they are capable of copying the DNA of living beings, they can be extremely dangerous.

In the comic books, Cassandra Nova has copied the DNA of Charles Xavier, causing her to become a powerful, bodiless telepath. She could also create bodies for herself, possess other living beings or operate whilst completely intangible "“ and that means she would make for a really interesting movie villain.