I may be overthinking this, but I think ideally, DAYS OF FUTURE PAST would cast the 60's X-Men into a future that takes place in an alternate timeline that takes place about the same time as the previous franchise.
Whatever the event the X-Men are going to have to stop from happening, the Brotherhood obviously needs to be involved. Whatever happens is going to cause humanity to persecute mutants, and to unleash Sentinels at some point in the future.
Here's the most important part:
With this more widespread persecution of mutants as a result of whatever threat the X-Men need to stop, and the concept of Sentinels eventually in play, William Stryker's lone actions in X2 become unneccessary, so the Alkali Lake disaster need never have happened in this timeline, allowing Jean Grey to survive past the timeframe X2 would have taken place, and altering the course of the future, by eliminating the attempt at the cure, and the events surrounding Dark Phoenix, which means Cyclops and Xavier would also have survived, in theory.
That way, even if it's just in a few sequences, we could see a future that involves Xavier, Cyclops in the badass team leader capacity that everyone wants, Wolverine, Jean Grey, Storm, Rogue, Iceman, Colossus and Kitty Pryde. Beast could easily be involved, given his history with the team. Angel could be involved, I suppose, as I would imagine in a world of persecuted mutants, he would have eventually found his way to the X-Men.
Given that all mutants were being hunted, we could possibly even see McKellan's Magneto, the original franchise's Mystique and Pyro working alongside the X-Men.
A last stand sequence could show James Marsden's Cyclops as a true field leader and the powerhouse fans have wanted to see for years, albeit probably going down fighting. We'd get to see the "real" X-Men one last time.
And then the lion's share of the story could still involve the fledgling X-Men and Brotherhood, with Xavier, Banshee, Havoc and Beast, trying to stop the actions of Magneto and his brotherhood that would lead to this disastrous future.
The more I think about the possibilities, the more excited I become.