2) Mr. Sinister
One of the last of the great X-Men big bads yet to make it to screen after Apocalypse, Mr. Sinister really needs to be the major villain of whatever the next X-Men film is.
A Victorian scientist obsessed with evolution, Sinister made a pact with Apocalypse to make himself immortal. By the time the X-Men are around, he is obsessed with experimenting on mutants and also has ties to the Summers and Grey families.
In probably his most notorious storyline, Mutant Massacre, he uses his team of Marauders in an attempt to wipe out the Morlocks, visible-deformed mutants who have hidden themselves underground.
The main reason for featuring him in the next movie is that a great actor has expressed interest in the part. At the 2015 New York Comic-Con, none other than Bryan Cranston said he would love to play Mr. Sinister.
Seriously, someone’s got to make that happen.