WARNING: The following article contains major spoilers for Deadpool 2, in theaters now.
Fans of X-Men comics will have recognized the name of the Essex House of Mutant Rehabilitation in Deadpool 2 as a reference to the classic villain Mister Sinister. It's not the first time Fox's X-Men franchise has teased the character's existence, as Nathaniel Essex's name also showed up at the very end of X-Men: Apocalypse, when men from Essex Corp. came to take genetic material from the Weapon X facility. While that post-credits scene stirred a lot of excitement among fans, Sinister has yet to appear.
By the end of Deadpool 2, we're introduced to X-Force, raising the question of which team of mutants Mister Sinister might face, if and when he finally shows up in the franchise. While not as critically successful as the Deadpool films, the core X-Men series has qualities that would benefit the exploration of a devious character like Sinister. Assuming that a future X-Force spinoff will share the same tone as Deadpool 2, a character like Mister Sinister might benefit from a touch of dark humor injected into the weightier themes associated with him.
Mister Sinister was introduced in 1987, in Marvel's Uncanny X-Men #221 as the enigmatic leader of the Marauders, a group of lethal mutants. Over time, every dark aspect of Mister Sinister was revealed: He was born in the 18th century and, for most of his life, has fed a twisted obsession with Darwin's theory of evolution, which drove him to conduct sick experiments, mostly on unwilling participants -- the exhumed corpse of his miscarried son, for example.
After being turned into a mutant by Apocalypse, who saw value in Essex's genius, Sinister continued to acquire other mutant powers through genetic tampering. Aside from superhuman physical capabilities, Sinister possesses a wide array of psionic, shape-shifting and regenerative powers. He's also seemingly immortal, which is how he's able to experiment on entire bloodlines, as he did with Cyclops and Jean Grey, resulting in the birth of Cable through a complicated scheme involving cloning ... and a lot of stalking.
So which film series is better equipped for a character whose purpose has never been to conquer or destroy the world, but rather to perfect it? That goal is similar to Apocalypse's, of course, only he reshaped Earth without finesse or, arguably, any real conviction. He occasionally mentioned "survival of the fittest," but at his core, he was more or less the typical world-threatening villain audiences have seen a thousand times already.
It's clear the X-Men films are capable of presenting villains of depth and complexity; the most prominent example, Magneto, has become a multidimensional character. From the very beginning, he was shown to be driven by more than just a desire to rid Earth of humans. The films made it clear that, as an antagonist, he represented mutants that had been wronged by humanity, and the other side Professor Xavier's philosophy.