Spoilers for X-Men: Apocalypse ahead!
Ah, the post-credits scene. Now a time-honored tradition, the post-credits scene has become a comicbook movie staple. Sometimes it's a funny moment, sometimes it's a big dramatic surprise. Most of the time, it's used to setup the future of the franchise.
In X-Men: Apocalypse's case, it's definitely the latter. As a man tours the Weapon X facility, which saw its staff utterly massacred by an unexpectedly released - and freshly engineered - Wolverine, he takes a vial of the subject's blood, and puts it in a case: Essex Corp.
So what does that mean, and what will it lead to in the future? Well, Essex is the real name of one Mister Sinister, a longtime X-villain with genetic engineering on the brain. A doctor in the nineteenth century, Essex was genetically altered by none other than Apocalypse (see the link?!?!), who gave him phenomenal powers and boosted an already brilliant mind. Armed with knowledge, seemingly eternal life and a body that can regenerate from nearly anything, Essex set out to help mutantkind flourish.
Typically, Essex has been linked most closely to two families in the X-Men universe: the Greys and the Summers family. Essex found in their genetic codes over a century before either Jean Grey or Scott Summers were born the potential for an ultimate mutant - and, secretly, one that could defeat and destroy Apocalypse for good. He set about manipulating the families, trying to eventually guide them together, and it worked - eventually, through some other crazy comicbook twists (it involves a clone, then an alternate timeline, and time travel...), we get Nathan Christopher Charles Summers aka Cable.
Essex or Sinister being revealed in any way in this universe is likely going to lead to one or both of two scenarios. First, he'll clearly be used in some way in the still untitled Wolverine sequel that is shooting now. There was a "mad scientist" character cast that we've been speculating for months could be Mister Sinister, and this scene certainly points in that direction. The film, said to be based loosely on the "Old Man Logan" comicbook storyline, needs a substitute big bad, after all - in the comics it was Red Skull, and his movie rights belong to Marvel Studios, not 20th Century Fox. Sinister has been linked to Wolverine and the Weapon X program in the past as well, and it brings in the possibility of Wolverine's clones or other subjects from the Weapon X program making their way into that film.
The other is of course Deadpool 2, which we already know will be introducing Cable. Cable has typically been inextricably linked to both Mister Sinister and Apocalypse, so having the pair of villains both introduced relatively shortly before the hero comes onto the big screen would be one heck of a coincidence, if that's all it was. With a new younger Jean Grey and Cyclops to experiment on (and a Jean who's reaching her full mutant potential much earlier than the original timeline, no less), now would be a great time for Sinister to step in.
We won't know for sure what role Mr. Sinister has in the future of the X-Men movie universe until those films start to come out, but the potential - like the potential of the mutant genome - is nearly infinite.