Seeing as how X-men and the upcoming movie Deadpool are owned by Twentieth Century Fox, both set to be released in 2016, it's no wonder this could be a thing we will hear in the upcoming year or so.
Deadpool: February, 2016
[X-Men: Apocalypse](movie:1194267): May 2016
After the success of X-Men: Days of Future Past, another X-Men movie is on the way, X-Men: Apocalypse. This will feature En Sabah Nur... or Apocalypse. From the end credit scene of DoFP, it's obvious Apocalypse will be front and center in the next movie.
In the comics, Apocalypse has been watching the whole time, and the altered timeline, is what gets him to attack. So now that Wolverine has gone back and altered the past, bringing back every X-Men character that died under Bryan Singer's watch, Apocalypse will be waking up.
Deadpool as everyone knows is the Merc with the Mouth also known as Wade Wilson (a DC Nod). Deadpool first appeared in 1991's New Mutants #98 created by Rob Liefeld and Fabian Nicieza.
In the comics, there is a version of Deadpool, renamed Dead Man Wade, where he is a member of the Pale Riders, a trio of assassins working for Apocalypse including Damask and Danielle Moonstar.
This version of Deadpool is seen in AGE OF APOCALYPSE, in the comics, this story is based on Legion, Professor Xavier's son, going back in time to kill Magneto. However, Magneto and Professor X are friends when Legion goes back in time and soon Xavier dies trying to protect Magneto and Legion is no more and a new timeline is created. Apocalypse was monitoring this and then decides to attack.
The only mutant aware of the altered timeline in the comics is Bishop, and in the movie it will be Wolverine.
Just like in the movie Days of Future Past, a new timeline has been created and Apocalypse will no doubt now want to take the earth.
Also, seeing as how Deadpool will be coming out in February, and the X-Men movie will be coming out in May, it's not unlikely Twentieth Century Fox will try to tie the two into the X-Men movie.
The February movie could be the origin story of Deadpool (which I think is unnecessary, they should just jump right into it with him) and then Deadpool appearing in X-Men Apocalypse in May as an assassin.
So be prepared, we could be seeing Deadpool in X-Men: Apocalypse.