In a recent interview, Bryan Singer told Fandango.com two important pieces of information. First is that each X-Men film since First Class will take place about 10 years from the last. And then the second thing he talked about is the strong alien presence that exists in the X-Men comics and how the next film may take place in space.
First Class was set in 1962. Days of Future Past was 1973. Apocalypse is set roughly in 1983 so that would put whatever's next in the early 1990's.
So which X-Men stories would work in the 1990s with all the characters being 10 years older and involves aliens and possibly space? Two come to mind immediately – The Brood and Dark Phoenix.
Let's start with the Phoenix / Dark Phoenix Saga. It involves the Shi'ar, the Starjammers and the Imperial Guard. So that's aliens and space. But it's a huge story to do right. They tried to cram it into X-Men: Last Stand and it didn't work well. As much as fans love this story, it may have a stench of failure on it too strong to get the studio behind doing again. Plus there are a lot of new characters to introduce, putting them in space for the first time and still having to deal with the concept of the Phoenix.
Now the Brood Saga would be a lot easier. You aren't introducing new characters, just an alien race that are almost all the same. This would be like making X-Men vs Aliens. The Brood are very similar to the Xenomorphs, which means fans will get them very quickly. But… the fact that they are very similar to the Xenomorphs may also play against them. But given a good script, this could be the strongest candidate in my opinion.
Other possibilities would be:
Free Charley – The X-Men go up against Warskrulls to free a kidnapped Charles Xavier and restore peace to the Shi'ar Empire. Fox might have the rights to the Skrulls along with the Fantastic Four.
The Rise & Fall of the Shi'ar Empire – This is a chance to introduce Cyclops and Havok's brother Vulcan.
Where No X-Man Has Gone Before – The team battle the Phalanx in an asteroid belt.