Diehard X-Men fans have been waiting for the moment teased at the end of X-Men: Apocalypse for a long time. It was in that post-credits scene that Mister Sinister, one of the mutants' most enduring foes was finally introduced, or at least those working for him were. It seemed likely that Sinister was being set up as the villain for whatever the next team X-Men movie would be, but comments from Bryan Singer and Simon Kinberg reveal that he will appear somewhere else first.
According to Cinemablend, the X-Men: Apocalypse audio commentary has Singer and Kinberg confirming the Sinister tease is a set up for James Mangold's Wolverine 3, and NOT the next X-Men movie. They don't go into further detail, but we can speculate based on Sinister's history. If you don't recall the teaser, it basically had scientists from the Essex Corporation retrieving a vial of Wolverine's blood from the destroyed Weapon X facility. 'Essex' is the commonly used name of Nathaniel Essex aka Mister Sinister, whose entire reason for being is to explore the genetic potential in mutants. More often than not he does this by cloning, and with rumors that we could see the debut of X-23, the female genetic clone of Wolverine, it stands to reason Sinister will be involved.
That also means the Sinister role has probably already been cast, and the most likely suspect is Richard E. Grant who was signed to play a "mad scientist". Sounds about right. Now keep in mind that Sinister is rarely the main villain of any story. He's usually a background player pulling the strings of others. He has an army of clone minions at his disposal, particularly the Marauders or Nasty Boys, who we could see Wolverine slice up. Sinister's ties to the Summers clan (Cyclops, Havok, and Cable), Jean Grey (he created the Madelyne Pryor clone), and Gambit (long story) also means we could see the villain across multiple movies. It'd be shocking if we didn't.
So we'll see how this all shakes out but I'm pumped to see what they do with Sinister, who is probably the X-Men's best villain other than Magneto. Wolverine 3 hits theaters on March 3rd 2017.