Most concise theory I've read
"listen, i'm gonna keep this short. the case is the play. if you read it, it drives you mad, and you fuging die. hart and cohle are just one pair of investigators that have fallen victim to it. detectives key and peele are at the beginning of their own investigation (visiting the church with the black preacher) right now. they're the next to go mad. how many sets of dudes fell prey to this poo before hart and cohle? plenty. because time is a flat circle. the play/case has always existed. it will always exist, and it will always kill those who begin reading it. that's why it exists, to drive people mad. just like this show."
I have to say, all things considered, I think that is how it is going to play out, I just would've written that out in like 8 more paragraphs, lol.
I was going to post my own theory that either Marty and/or his ex-wife is in on it, OR Marty is not in on it and either he or Rust are going to die in the finale. I came to this conclusion after Marty's lack of conviction when asked why he left the force and because of the screwed up stuff with their oldest daughter. And his wife has been suspect for awhile. The way she used Rust was downright evil. And for some reason, I kept thinking back to that initial argument Marty was having with her dad about society back in the first or second episode... there was just something about it.
BUT, as the prediction you just posted mentioned, what really kept standing out to me was Rust saying the thing about his life is one big "circle" and the "death is not the end" stuff that the lady told him in this most recent episode. Which again, goes back to when Ledoux told him life was a flat circle, to which Rust quoted to the investigators... And at the end, the two new investigators drive off and the man with the scars on his face says, "My family's been around a LONG TIME."
I assume that is implying that they are somewhat immortal, and that they may feed off the souls that get trapped/lost in Carcosa.