Re: Why not do both? X4/First Class... Combine The Franchises!
I think its a novel idea, however I would leave out the Days of the Future past aspect. We all love the classic stories from Claremont's golden era, but it really doesn't work with the movies.
Now, it COULD work, if it were tweaked a little bit. You could still use a "past" cast and a "future" cast, with the assassination of a politician as the fulcrum of the story. Rather than time travel though, a mystery/thriller element would be used, where we see the "first class" era laying the ground work for what transpires in the future, and the "future" era needs to dig up the past to unveil the killer(s). Think an episode of Lost or something, where flashbacks are blended with the present with relevant pot details that culminate to a climax or shocking reveal.
Actually, I REALLY like that idea. Its a good way of blending old and new, could still use the classic name, and would be a cool new spin on the story.