The scrapped X-Men: Dark Phoenix in 2019, Fox's X-Men franchise had already presented a wide variety of X-Men villains, including Magneto, Apocalypse, and the Sentinels. Ss extensive as it was, the almost 20-year-long franchise barely scratched the surface when it comes to mutant lore.
One of the key X-Men villains who didn't get to appear in New Mutants' Dr. Reyes is shown to be working for Essex Corporation.
How Fear The Beast Used Mr. Sinister
Byron Burton's draft for X-Men: Fear The Beast would have finally showed Mr. Sinister's face for the first time. In Burton's script, Hank McCoy faces Dr. Paul Cartier, a scientist who transforms the antidote to Beast's transformation into a weapon. While Beast confronts Cartier and runs into a post-Weapon X Wolverine, a disfigured man follows his tracks. At the very end, the so-called "Scarred Man" retrieves Dr. Cartier's weaponized serum, which heals the mysterious man's wounds and reveals his true identity: Mr. Sinister.
Curiously, X-Men: Fear the Beast would have added yet another tease to Mr. Sinister's cinematic history. Yes, Nathaniel Essex would have finally appeared onscreen, but his actual role as a main villain would have had to wait for a future sequel. In the meantime, Fear the Beast would have teased Mr. Sinister's motivations. Given that he keeps a close eye on his modified antidote in the Fear the Beast draft, it seems that Mr. Sinister aims to enhance his body with artificial mutations, after which he would likely set his sights on transforming regular humans into mutants in a plot similar to the MCU's overarching Super Soldier serum plotline.
Why Fox's X-Men Films Became Focused On Mr. Sinister
Magneto, Mystique, and Jean Grey were a key part of Fox's X-Men franchise, as they were villains as much as they were heroes. However, the X-Men movies never had a franchise-wide villain that guided each film towards a specific conclusion. Although late in the game, Mr. Sinister could have filled that role, with X-Men: Apocalypse, Deadpool 2, and New Mutants laying the groundwork for his official debut X-Men: Fear the Beast and his subsequent main villain role in a future movie. Mr. Sinister's Machiavellian nature makes him an ideal character for this kind of role, as his tendency to come up with shady schemes is patent in the comics.
Now that the MCU is incorporating parts of Fox's X-Men franchise, perhaps Mr. Sinister could become the MCU's first X-Men villain. The scrapped X-Men spinoff X-Men: Fear the Beast could provide some inspiration, but it will probably remain a discarded script. While previous X-Men movies teased his eventual debut, Marvel Studios can introduce Nathaniel Essex without the need to pay off Fox's previous set-ups.
