10 Reasons X-Men Should Never Join The MCU
9. The Quicksilver/Scarlet Witch Issue
One of the small but glaring details that would prevent or needlessly complicate an MCU/X-Men integration stems from how each respective universe has handled the Maximoff twins.
In the X-Men film universe, Quicksilver is a kleptomaniac mutant with an affinity for band t-shirts who lived in his single mother's basement prior to joining the X-Men proper. The standard story of him being Magneto's estrange son is used, with the existence of Scarlet Witch being rather vague (it's shown that he has a sister, though she is clearly much younger than him and her name is never revealed).
Meanwhile, without the rights to Magneto or their original status as mutants, the MCU turned the Maximoffs in to Hydra lab rats whose unnamed parents had perished when they were children, before they jumped ship to the Avengers and Quicksilver met an untimely death of his own.
Fox and Marvel likely didn't think that much about a potential collaboration back when they agreed to share the rights to the Maximoff twins and as a result, the two X-Men characters already in the Marvel universe, ironically, stand in the way of any more coming over. You can bet this would be a massive elephant in the room for fans should Magneto and co. find their way to Marvel.