So each fandom has their own worlds right? When there is a fanfiction one fandom goes to another world. For example:
Percy goes to Hogwarts. Not the other way around. Harry doesn't go to CHB. Each fanfic is in the right world.
Percy- Hogwarts
Maximum Ride- CHB
PJO- Mortal Instruments
This may sound really weird to you, but it all makes sense in my fangirl mind.