A few thoughts I have:
I hope they maybe introduce a few new characters as horsemen/use existing characters not known for being horsemen. I do think Archangel is a must for a horsemen.
In a future timeline there are a lot more characters to work with.
Death:
Archangel
War:
Wolverine/Colossus
Famine:
Rogue (giving her the ability of super strength and flight)
Pestilence:
Caliban
Looking at a 1980's timeline I really like how the Horsemen played out in X-Men Evolution. I would be ok with Magneto, Xavier or Mystique becoming Horsemen. I also loved the way the Horsemen looked in this series. Dark Beast may also work.
The problem with the current first class timeline is that there seem to be only a few characters remaining. Aside from Beast, Havok, Mystique, Quicksilver, Xavier, Toad and Magneto (and a couple of other mutants throughout the film), there isn't much to build a team of 4 Horsemen that would be able to match up against the remaining protagonists. Characters will need to be introduced without stepping on the eggshells of continuity this franchise has been working on repairing. At this point in time Wolverine has no memory of X-Men DOFP and would at this time be working with Stryker presumably. If they decide they want to use Wolverine, they will have to go ahead with sending Wolverine back in time again, or find a way to work him into the story line post DOFP amnesia and pre Weapon X amnesia.
As for Mr. Sinister... I would be ok with seeing him in this movie, but only if he wasn't a Horsemen. If he is included, I would like to see him as the man behind the "resurrection" of Apocalypse, or as a second in command with a hand in genetically modifying and enhancing the Horsemen's powers.