I'm working on a story right now that has put me in a somewhat more difficult position writing than I think I've been in before. It's about dreams -- specifically, it's a simple inversion of the assumption that "awake" is the chunk of time when stuff makes sense and follows continuity, and "dreaming" is the time when everything is weird and symbolic. This idea seems like it should be pretty easy to work with, but it very quickly became almost completely unmanageable. I began to find that nearly none of the structural assumptions that help make sense of the world apply in even the remotest sense.
In this narrative, humans aren't mildly irrational creatures in a naturalistic world. They're entities so logical that, in an idealist, formal world, we share a species-wide hallucination that seeks to make sense of the disjointed input of reality.
It's fun to explore, but it's also really complicated. Everything has to bear a relationship to reality and to dreams, but for a character who understands it, it would have to make an entirely disconnected but overlapping sort of sense.
I haven't figured out what that kind of sense looks like. But I hope I do, because if I manage it, this is going to make a really cool story.