This one is one that I've recently been noticing lately, and I guess it's one that's a little hard to go around depending on the fandom.

For each fandom, there seems to be only one or two used villains in their fanfics. No one seems to take the time to make an OC villain or a villain that hasn't been used so much that it's a cliche. Here's a chart of different villains in fanfics:

Harry Potter - Voldemort... Always Voldemort
Percy Jackson - Medusa or Kronos
The Beatles - Almost every one I read that was based off of when they were still together had Jane Asher as the villain if it's a romance with Paul. If it's George, then it's Pattie. And I'm not talking like they are in the way of the romance; they are portrayed as the most horrible people, and it gets very annoying and it's really rude to them.
The Monkees - Mr. Babbitt (hope I spelled that right), Harold, or the company that made the show themselves.
Avengers - Thanos or Loki

There are more than just a few options per fandom to have as your antagonist. Try new things! What if you used an OC if you cannot think of anything?

Thank you for coming to my TED talk,
- Ellie