Something about the afterlife.
The idea your afterlife is whatever you spend your life believing it'll be (or something like that).
What if you don't believe in an afterlife, and you get there, and there's nothing there, but you're still conscious of it?
The end of The Sirens of Titan and the secret plot twist that defines the entirety of Mulholland Drive. "Your afterlife" is a crazed hallucination experienced in a split second before your brain--and your consciousness--expires. But this is boring, and also HORRIFYING. What if the life of a soul (as I have suggested for almost a decade at this point) is so much less defined and determinate (at least when contrasted against the corporeal and the mortal) that it's like a bunch of mingling dreams like when a bunch of GMs clash their worlds together in those crossovers I used to take so much part in when I was at PDS.
Life, and the body, then, just become moments and means of participation in a system with common rules that have been established by no one of us.
...My mind is FALLING APART. I feel like I can't form a coherent sentence. I'm not even sure I remember where I was going with this.
Edit: Also, What Dreams May Come. And that story about the afterlife that Conor started to write. ...Maybe we just drift from life to life. The actions you take in one life determine what the next will be--almost every religion suggests this, but I'm referring to the religious decisions you make in life... like, if you sign up for Christianity or whatever, your actions will determine whether you'll go to Heaven or Hell, meaning your life then will take place in a world whose rules were written by Christian God or Christian Devil. This is only interesting because now I wonder who wrote the rules for this life. And what I did in my last life to wind up... yep. I can't form a sentence. Sorry, y'all.
Edit: It's also a little like Wayfinder (as all of my understandings of lives outside of this one are) in terms of story writer. You might know what I mean.