I'll miss all the "spooky" communities, they are what brought me to Reddit in the first place way (way, way...) back in the day. Nosleep, creepy, aliens, thetruthishere, etc. There's lots of them, of varying quality these days but I regularly enjoyed the discussions and theorizing over the years.
Shout out to unresolved mysteries, that was a long term favorite of mine. At its best it was a respectful and helpful community of true crime researchers working to bring attention to cases. I am truly fascinated in evaluations of cases of misadventure, as well.
I might miss the movie discussions most, though. I love nothing more than picking apart an interesting story and getting under the obvious themes. I would regularly go directly to the official movie discussion threads after watching something I enjoyed, then search posts about it as well. I hope to find others here with the same interest because I'm sure that is something that can continue in the fediverse.