The post-credit scene of X-Men Apocalypse teased the introduction of a one Nathaniel Essex, who is better known as Mister Sinister. Mister Sinister was one of the most powerful villains in the X-Men comics. He was driven by an obsessive desire to understand the genetics of mutants, and he would regularly experiment on them as test subjects until he all but achieved immortality for himself.
Many out there are hopeful right now that we'll be looking at Richard E. Grant as Mister Sinister, after news broke a while ago about Grant being cast as a 'mad scientist.'
Wolverine 3 comes out in March 2017.