I don’t know what other people collect, I’m all over the place with it myself. Mostly I collect books though, whatever my current interest is, I consume all the media related to it.
Then there’s magic (as in stage) history and mechanics, I have a modest collection of books on the subject. Pulp novels and comics are part of my library but my comics aren’t a serious collection, more highlights of what I like.
My longest running collection of objects is Pranks. I don’t particularly like pulling them but find them strangely fascinating to think about. Then there’s a bunch of escape artist stuff, chains, manacles, mailbags. All tied into my interest in stage magic. Not a magician to be clear and I don’t practice but I am knowledgeable about it in my own way. I like to know how things work.
Batman and his villains specifically Joker themed things. Each have there own place opposite each other, with some Riddler stuff to.
Aliens has filled its own section of the library between when I was playing Alien: Isolation and waiting for Predator stuff to be delivered.
Horror movies and a few figurines make up the rest of what I collect or at least the sections large enough to have there own category.
Like I said it’s kinda all over the place. I even have a full sized arcade crane game in my living room, filled with stuffed animals for my Fiancé. I’d collect more if space and money, were no problem.
Edit - Collect whatever makes you happy man, that’s what I do. I love horror, asymmetrical game’s and the things connected to them. If I’m curious about something I learn about it and add it to the collection. One day I hope to have a ‘Cabinet of Curiosity’, a room of my interests.