While I'll be surprised if they end up going with the
Days of Future Past storyline for the
First Class sequel, I will admit that seeing Michael Fassbender and Ian McKellen possibly sharing the same frame would be too awesome for words.
Plus, introducing the storyline in the sequel allows the filmmakers a better opportunity to tie this "prequel verse" with the Singer verse. It would also give everyone a chance to see some familiar faces... which I don't think anyone would be adverse to. I'd love to see Patrick Stewart, Ian McKellen, and others again.
Hell, this might be crazy, but perhaps the inclusion of this story could erase
X-Men: The Last Stand from continuity? Maybe that's being
too optimistic.