The existence of Mister Sinister was only hinted at in the X-Men movies, but it has been revealed that he was almost the villain in X-Men: First Class. That duty instead went to Sebastian Shaw and the Hellfire Club.
X-Men: First Class was a project that evolved from a solo movie that was originally centered around Magneto, which was going to be called X-Men Origins: Magneto. However, the Magneto movie never made it into production due to various scheduling and technical setbacks. And though X-Men Origins: Magneto would never be made, the elements of the movie were used in X-Men: First Class - most notably the content related to Magneto's backstory.
One of the writers of X-Men: First Class, Zack Stentz, recently appeared on The Fanboy Podcast in order to promote his upcoming Netflix film Rim of the World. It was during this interview that Stentz revealed that Mister Sinister was originally planned to be the villain of X-Men: First Class instead of Sebastian Shaw. Stentz spoke about the elements from the original script that made it into X-Men: First Class when he mentioned Mister Sinister's original role in the film. He said:
"There were scenes and pages from that original Magneto script that were folded into the finished production ... The opening in Auschwitz - and then with Sebastian Shaw - [was] right out of that original script, except that it was originally Mister Sinister, which is why Sebastian Shaw actually has powers more like Mister Sinister's than his own."
The comment referring to Sebastian Shaw having abilities similar to that of Mister Sinister refers to a change that was made to his established powers in the comic books. In the pages of Uncanny X-Men, Shaw could absorb kinetic energy to make himself temporarily stronger, while the X-Men: First Class version of Shaw could also use the energy to heal and sustain himself, which is similar in function to Mister Sinister's regeneration abilities.
It would have made a lot more sense if Mister Sinister had played the villain role in X-Men: First Class over Sebastian Shaw; especially the idea of him working with the Nazis in order to experiment on mutants, with Magneto taking Cyclops' place from the comics. It also makes sense why Riptide of the Marauders was chosen to be a member of the Hellfire Club in X-Men: First Class, as Mister Sinister was the founder of the Marauders in the X-Men comics.
Mister Sinister was the most high-profile X-Men villain to never make it into the movies. The movie rights to the X-Men franchise are now owned by Disney, and there will a come a point where the team enters the MCU, even if it won't be for a while. The fact that Mister Sinister remained untouched by the Fox movies means that there is a strong chance that he can be one of the first villains to be used when the X-Men finally appear in the MCU, especially as Bryan Cranston is still talking about wanting to play the role.
Source: The Fanboy Podcast
- X-Men: Dark Phoenix (2019) release date: Jun 07, 2019
- New Mutants (2020) release date: Apr 03, 2020