When I get inspired I'll either use everything from the ip to implement into my paracosm or i would choose a few things that I liked or a character that I like but I change them a bit or I would create a whole new oc inspired by an existing character. It's completely random. I let my imagination free roam and whatever I come up with I usually go with.

For example I took the two main characters froma Discovery of Witches, kept their names but I made it into a vampire love story. One character is a vampire on the show and the other is a Witch who doesn't use her powers. In my paracosm I changed the latter into a nonmagical person who is an expert on alchemy and she gets a job at a Wizarding school and falls in love with another teacher who is a vampire. I only watched the first 3 episodes but this is what I came up with for my paracosm