I have been pondering this as well because I have been writing in a TTRPG setting and I am pretty sure that if I use my original characters, that would be fine for mosaic. It gets fuzzy if I want to use non-player characters provided by the rulebook to interact with my characters. I still think that is probably fine? My character is a member of the Illustrious Qilin Villa sect (provided by the game), why couldn't he interact with the sect's masters (NPCs from the book)? For my conscience, I would probably try to keep the focus on my OC though.

It does create a situation where I can technically call it fanfic, and have my cake and eat it too. I have already been posting fiction like this on AO3 as fanfic for the game.