I say go with just one more X-Men using the characters and actors we have now. Age of Apocalypse would be perfect and they could, once again, use both casts. It may come off a bit "been there, done that"; but the difference is the future is totally different since it's the Age of Apocalypse.
In the past it would be really cool to see young Xavier sacrifice himself to save Mags. Yes, that means a very limited use of old Xavier, but still enough to throw him on the poster. The future changes and most of the movie goes between what is happening in the Age of Apocalypse and what needs to be fixed in the past. The story is different enough to DOFP, but it is still a timeline movie, but oh well.
After that our aging actors, along with all of the timeline issues, etc; will need a reboot. But this time they should just start fresh. What I mean by that is just go with the real first class of Cyclops, Angel, Beast, Iceman and Marvel Girl. This would be 5-6 years from now, so people will be ready for the reboot.
But this time, do it right. No using secondary characters that shouldn't even be there. Simply start with the fist class of 5 X-Men and have a couple of movies adding some (Havok and Polaris for example). Have them in a flick where Xavier has to recruit new mutants from around the world to save his X-Men. This would be the "New X-Men" with Wolverine, Storm, Banshee, Nightcrawler, Colossus, Sunfire and Thunderbird (hey, someone has to die...).
In other words, reboot with the 616 universe that we all love and would love to see on screen!