I would go with a different approach with the Horsemen. We had changes for the films, and by me they worked very well by now. For the Horsemen I would like to see a more serious approach than in the comics. I see them as phenomenons that occurred throughout our planets history.
First came the Famine, in the ancient times when man has established himself at the top of the food-chain, as civilizations flourished. Then.. when humanity survived and ascended in reason and intellect, Plague punished the cultural center of the world as the Black Death struck Europe. Humanity prevailed and even advanced in the fields of science, biology, physics, chemistry, astronomy.. The time was idle for the third Horseman to attack. The World War engulfed the whole world and War nearly succeeded in it's goal, but we remained. Now as the fourth Horseman prepares to strike in the form of Death Apocalypse will rise and devour the world, bringing an end to the evolution of humanity.
Of course the Horseman could have some forms during the film, but I wouldn't like some of the mutants to become them. Like I read some suggest Phoenix. That would ruin the symbolism between Apocalypse and Phoenix / Set and Horus. The Phoenix, finally resurrected like Horus did to avenge Osiris, will be Apocalypse's downfall after Angel opposes his master and refuses to finish the prophecy of the Fourth Horseman. The Angel could be Death and the story could revolve around his character until his final manifestation to Archangel.
Also I would take Angel to become the Horseman only if it is discovered that he possesses rejuvenating powers, which Apocalypse would alter and deform to the power of bringing death.
This is my idea for the 6th X-Men film, though I don't believe it will ever see the light of day. I also believe that you guys will not like it, but I took in consideration the movies that were previously done and followed a path the story could take.