I feel like a lot of the things you're doing don't actually have fandoms? They have fans but fans =/= fandom. Like for example Spider-Man, everyone loves Spider-Man, no argument there, but it doesn't really have a fandom per se. Wouldn't generalizing it in to like, a Marvel category be better? Same thing could be said about many others, like Invader Zim? There is really no fandom there. get separating the Avengers from Marvel because it has a huge fandom though.
I have a process with picking the fandoms. I read through as many asks as possible, and if a fandom shows up 2-3 times, I add it to the list of ones to be done. If it shows up 10-20 times, I put it on the list in bold. And then I just go down the list, trying to do the ones in bold first. I got at least 3 requests for a Spider-Man fandom poster, so I made a Spider-Man fandom poster. With the new movie, a lot of new fans came and made a fandom. I got at least 3 requests for DDR, so I made a DDR fandom poster. The fandom may be tiny, but they wanted a poster bad, so a lot of them sent in asks. I don't decide "Hey I think this fandom's big enough" because many of these I've never even heard of before, but there are apparently a lot of people in the fandom. I have rules to make things fair, and I follow them. That's it.