Surprise, surprise: we don't know anything about the theme! The best we can offer is a few solid theories about where the show could go next. The best bet right now is something involving space or aliens. Allow me to explain.
As diehard fans know, Murphy quite often drops hints about the next season in the current season. During Apocalypse, there are quite a few hints that seem to point to something involving the cosmos, space, or intelligent life in the universe.
- The inclusion of the Carpenters song, "Calling Occupants of Interplanetary Craft," seemed to be one of the most overt winks early on.
- The "Grays" in Outpost 3 were also perceived, by some, as a nod to the most common iteration of alien lifeforms, often called "grey aliens" and yes, sometimes simply "greys."
- Then there's Ms. Mead, who gives all sorts of hints! She sings "Daisy Bell" as her decapitated head slowly stops functioning. It's an homage to 2001: Space Odyssey; the smart computer HAL9000 sings the same song while shutting down for the final time. Speaking of HAL9000, Mead's creators also call her "Battleaxe 9000" at one point, and it's safe to say the number 9000 is more than random coincidence.
So, that's as much as we have in way of theme. If aliens are involved, though, it could be another way to foster more season connections. Perhaps even an Asylum sequel!