James Marsden, Halle Berry and more are in talks to return for
X-Men: Days of Future Past!
According to the link, many of the actors from the original trilogy are willing to return because of the goodwill established by Singer's involvement. So, as
Admiral_Young speculated, actors are returning mostly because of Singer's involvement. Which is music to my ears, and another reason why I knew Singer coming on-board to direct was a good thing for multiple reasons.
Gahhh, I'm so excited for this movie!