Marsden is on the record saying he would love to
play the role of Cyclops again, as are
Stewart for Prof. X and
Berry for Storm. Ashmore is still contracted to
play Iceman in a fourth film as well, and of course Hugh Jackman has said he'll play Wolverine
for as long as he can. I have no confirmation of this, but it's very likely cast members for
X-Men: The Last Stand were signed under multiple picture deals, so for instance Ellen Page appearing as Shadowcat or Kelsey Grammar playing an older Beast wouldn't be totally out of the question.
Other original actors however such as Famke Janssen and Anna Paguin are no longer under contract and
would be difficult to bring back to the franchise (but story-wise that's okay because Jean is dead and they can say the Cure was indeed permanent for some mutants, e.g. Rogue).
With such facts in mind, these are the characters I would like to feature in
X-Men: Days Of Future Past:
1970s Ensemble
James McAvoy as Prof. X
Nicholas Hoult as Beast
Lucas Till as Havok
Skyler Samuels as Magma
Ryan Kwanten as Cannonball
Michael Fassbender as Magneto
Jennifer Lawrence as Mystigue
Story-wise, I want this to take place 8-10 years after First Class; Banshee has left, while other mutants such as Magma and Cannonball have joined the team in that time. I want it to be implied that Magneto's brotherhood has grown, but I only want Mags himself and Mystigue to play any sort of role (NOT as the main villain; save that for the next film, and not at all in the future as he will be dead by then).
Present-day Ensemble
James Marsden as Cyclops
Halle Berry as Storm
Shawn Ashmore as an older Iceman
Josh Holloway as an older Gambit
Kelsey Grammar as an older Beast
Hugh Jackman as Wolverine
Story-wise, I want Cyclops reappearance to be tied into the time-travelling plot somehow (e.g. he's already back at the start of the movie, and later it's explained that Bishop saved him from the Phoenix and transported him to the future where he was needed). Wolverine needs to be a minor supporting character (he'll have left the X-men after Beast takes over as headmaster of the school, but be helping with the fight on a solo level).
Other (time-travelling)
Ellen Page as Shadowcat
Morris Chestnut as Bishop
Not entirely sure how I want the story to play out here, but Shadowcat and Bishop need to be the only characters that appear in both time periods (until the final battle where the teams potentially join forces to fight an army of sentinels; maybe in the future, before time is righted and the dystopian future disappears and everyone is returned to their respective periods). Again, I don't know what I want to be the historical trigger for the dystopia (or Sentinel Program if that's what leads to it), but for the main bulk of the story Shadowcat and Bishop must be the only characters that travel between time periods in order to keep the perspectives (for the audience) clear.