So "fandemonym" is a new word that I coined that the English Language totally needs. Allow me to explain.

A fandom, as you surely know, is a group of fans, almost an entity unto itself. A demonym is the word for the people who inhabit a specific place, such as Washingtonians, Americans, Chinese, etc.

Therefore, a fandemonym is the word for the people who "inhabit" a fandom. Sherlockians, Whovians, Homestucks, Night Valiens ( yes, with an e. i don't think this is a thing yet, but it totally should be)