I think a story involving Sinister and Hellfire Club might work. I'd also like to see Sentinels, Proteus, and some version of Apocalypse.
I'm not sure if some dark, horrific future wasteland story like Age of Apocalypse would work, because it's often said that movies set in bleak, dystopian futures do not tend to do well because the audience cannot connect to them.
Given what we've had already, the story needs to take place in a version of our world.
If you want anything really fantastical like space voyages, dystopian futures, aliens, hidden prehistoric lands, cosmic crystals or living islands, then it probably has to be a reboot that has a more fantasy setting from the beginning.