Re: First Class 3 or X-Men 5?
A sequel to First Class in a new timeline is just as effective as a reboot. That way you still get to keep Singers two original movies (and third DOFP) yet it gives them the freedom to start on a new trajectory with characters.
They could essentially introduce characters again such as Cyclops, Storm, and Jean in a new timeline and not be bogged down by what has happened in the past movies. Plus the franchise can simply last longer. There's only so many movies they can do with the original cast - heck I even think one more is pushing it.
It's going to eventually become stale. Might as well start anew now with the awesome First Class cast they have now and they can only build off of that with additional casting as well.