Jackman is currently in talks to reprise as Wolverine, with McKellan excited to be playing Magneto again. Stewart recently noted that he "hasn't heard" anything about playing Professor Xavier, and now there is a hint that more original X-Men characters might be returning for the new movie.
Regarding other additional characters, THR notes that none are confirmed, but mentions Halle Berry and James Marsden, with further mention that the actors that are involved are eager to return because they are so fond of Singer.
Bryan Singer recently came on board to direct X-Men: Days of Future Past after Matthew Vaughn left for Mark Millar's Secret Service movie, or even possibly the new Star Wars.
X-Men: Days of Future Past has a July 18, 2014 release date.