There are dozens of series/franchises like the MCU, the Trek Federation, or the Shire of the Hobbit. Ones that already have an immersive folklore.

A Buffy/Charmed/Supernatural crossover world would be pretty easy to conceive, but unless you were a protagonist you’d probably be a victim to a hot Goth vampire.*

*Yes, I know that there are plenty of people here who dream of being slain by a hot Goth vampire.