The X-Men: Apocalypse post-credit scene (watch below) features the aftermath of the Wolverine berserker rage scene with the Weapon X blood being cleaned up and placed into a briefcase, which is labeled Essex Corporation. In the comics, Nathaniel Essex is Mr. Sinister.
According to Cinema Blend, audio commentary on the X-Men: Apocalypse digital release features Bryan Singer and Simon Kinberg confirming Mr. Sinister for Wolverine 3. It's said Kinberg and Singer discuss the Mister Sinister post-credit scene as a way of preparing audiences for the third Wolverine movie.
It's now speculated Richard E. Grant may be playing the role of Mr. Sinister in Wolverine 3.
The untitled "Wolverine" 3 has a March 3 2017 release directed by James Mangold also starring Elizabeth Rodriguez, Boyd Holbrook, Stephen Merchant, Doris Morgado and Eriq La Salle.
X-Men: Apocalypse Mr. Sinister Post-Credit Scene: