I honestly believe Fox/Singer don't know what they are doing with this franchise after DoFP. I think they've tossed around some ideas, but nothing is settled. They want to keep their options open. That's why there is a good chance Famke and Marsden make cameos at the end, further convoluting things in the timeline, but presenting an avenue for their return. They want to keep their options open for the story to go in a number of directions, but there won't be a definitive direction given the uncertainty with the cast/story going forward. That's why X-Force is a potential stop gap, and offers audiences a break from the usual.
Originally Posted by BMM
It's possible Marsden would have had a larger role, if he hadn't signed on to Returns, but it's hard to say how big that role would have been, especially with Singer gone and Fox at its worst. Fox didn't even want to follow up on the Phoenix storyline. Rothman had to be convinced by Kinberg and Penn to do it, if that gives you any idea how completely stupid the studio was at the time.
Actually, Rothman was right for the wrong reasons. Phoenix could have been saved for X4, and Singer wanted to do two films back to back before SR. Obviously Rothman wasn't thinking about X-4 at the time, but at least the Phoenix would not have been butchered had they just focused on the cure on the battle without Phoenix, and maybe they would have kept on Ratner to do X4, or Singer may have come around, given that SR was a financial disappointment. Imagine that? We wouldn't be talking about Origins or The Wolverine if that were the case.