"X-Men Apocalypse" may choose to follow the storyline "The Twelve." The comic based plot can give us valuable clues about the possible cast.

Months before "X-Men: Days of Future Past" is released, director Bryan Singer has already announced the sequel, "X-Men:Apocalypse." It is easy to know who the villain is, but who among the X-Men past, present and future will be around for the next movie? If they follow the Apocalypse crossover storyline in the comics "The Twelve," we can make a list.

One interesting feature about Apocalypse is that he always has "Four Horsemen." His horsemen were powerful mutants and he would name them according the Four Horsemen in the biblical reference: War, Famine, Pestilence, Death.  The roster has changed from time to time, and has included some of the X-Men at some point. Apocalypse manipulates the mutants he recruits and also augments their powers.

Wolverine and Angel (then known as Archangel) Gambit and Psylocke were once The Horseman of Death.

Polaris (Lorna Dane, Havok's fiancée at one point) was once the Horseman of Pestilence. Polaris has the power to utilize magnetism similar to Magneto, which was discovered to be her biological father (Spoiler). That could be an interesting plotline.

Even The Hulk became the Horseman of War, but his rights belong to Marvel.

Finally, there is "The Twelve." These are considered as the most powerful mutants and predicted to bring the mutant supremacy over humans. Apocalypse sought to gather all of the Twelve and absorb their powers to give him godlike status.

From Marvel Wiki, the Twelve and their representations:

"Magneto and Polaris, representing opposing magnetic poles.

Storm, Sunfire and Iceman, representing the elements (with Storm evolving into a primal elemental force).

Cyclops, Phoenix and Cable, representing family (Father, Mother, Child), and chosen for the power of the Summers-Grey bloodline.

Bishop and Mikhail Rasputin, representing time and space, respectively.

Professor X, representing the mind.

The Living Monolith as the core."

As always, it is not certain whether the movie version will follow the same roster. It may even depend on which actors they can sign. Most of these mutants have already appeared in First Class or the X-Men trilogy, and some will be introduced in Days of Future Past (Bishop, Sunfire).

The Living Monolith, Cable and Mikhail Rasputin, all of which are key elements of the next X-Men phase, have yet to be cast.