The potential Disney-Fox deal certainly threw a complicated wrench into the works of Fox's plans for the X-Men movies, but until the deal is done, the studio has to keep on trucking and putting movies out there. Deadpool 2 just recently released in theaters with the second highest debut for an R-rated movie, while New Mutants and X-Men: Dark Phoenix are set for sometime in 2019. The studio has announced some superhero projects are in development and while the main X-Men series is still up in the air, Fox may be teeing up the debut of a classic X-Men baddie to plague the franchise: Mister Sinister.
For those who may not know, Mister Sinister is a somewhat obscure, dumbly named, badly dressed X-Men villain who is actually really messed up and terrifying. Born in the 18th century, Sinister was a scientist obsessed with Darwinism and evolution. He even predicted that eventually humanity would evolve into super beings, but his theories were mocked. This eventually caused him to cross paths with Apocalypse, which gave Sinister powers of his own. Now immortal with a host of other powers, Sinister experimented and tortured mutants for generations. He eventually became obsessed with Cyclops' bloodline, stalking and manipulating him for years. Sinister even created a clone of Jean Grey and is ultimately responsible for the birth of Cable.
While some X-Men villains have shown up multiple times now, Sinister is one of the only remaining major X-Men villains who has yet to appear in any of the movies... but that might not be an entirely true statement. Sinister has actually been name dropped a few times now throughout a couple of different movies, but only really in an easter egg capacity that wouldn't mean much to people unfamiliar with the character.
Sinister's real name is Nathaniel Essex, and that last name should be fresh on the minds of people who just got out of Deadpool 2.
The name Essex first showed up in the post-credits stinger for X-Men: Apocalypse. After Wolverine's rampage, he leaves behind plenty of blood, some of it his, most of it not. Mysterious men arrive on the scene and begin to collect Wolverine's blood, placing the vial into a suitcase marked Essex Corp. This would imply that Mister Sinister is collecting Wolverine's DNA and people assumed this meant the villain was finally up to bat.
At the time, people thought that this was set up for Logan, which involved a clone of Wolverine and young mutant children experiments. All of that is kind of Sinister's M.O., and while he was rumored to be in the film, that was ultimately not the case. There was a mad scientist-type as the villain, but he wasn't related to Sinister in any way.
Sinister's name most recently popped up in Deadpool 2 and this time it was a little bit more prominent. The name "Essex" adorned a mutant orphanage that doubled as a mutant re-education center. Once again, scores of mutant children were left traumatized by twisted science, but there was no overt call out to the character himself. The names Nathan or Mister Sinister are never spoken, but this is the second time Essex has popped up in the X-Men universe, so something must be planned with the character. Right?
It's unclear what the plan is for Sinister, but it seems like there are two avenues. He can either appear in the X-Men film that comes after Dark Phoenix (which is likely too crowded for him to show up), or he could be the main villain in X-Force. Either possibility could work, but Sinister might actually benefit best from appearing in X-Force.
Sinister is a really twisted character, and he might be better served in the potentially R-rated X-Force, which doesn't have to hold back the horrors for a PG-13 rating. Plus, Sinister has ties to the main three members of X-Force: Deadpool, Cable, and Domino. Deadpool was none too happy about what was going on in the orphanage, while Domino was actually raised there. Cable has no revealed connection in the movie, but as stated before, Sinister is responsible for Cable's birth, so that option exists. X-Force is a total unknown at this point, but Drew Goddard will direct it. Sinister's involvement likely depends on if X-Force keeps Deadpool's emphasis on humor, or if it sways closer to the seriousness of the X-Men.
The X-Men really don't have many good villains left and before Fox starts scrapping the bottom of the barrel with someone like Arcade (some knob job who traps people in murderous carnival games), then Mister Sinister is the next best option. Maybe these hints are just easter eggs and it certainly wouldn't the first time an X-Men movie teased a cliffhanger and then completely abandoned it. However, these name drops seem a little too important to just be throw away easter eggs.