I think for such a thing to work it depends heavily on how intertwined with the worlds lore and mechanics. A big reason why fanfiction itself works well is because a writer/artist doesn't have to invent these things themselves; simply build new stories with the existing characters or insert their own into those worlds. Then there's au's/alternatue universe stories that either take simple changes to characters roles (like coffee shop or college etc) and then you have those which go through near complete alterations to worldbuilding structure to make them into something similar but unique in their own way
Like I'm a long time Sonic fan so I've seen a broad array of sonic fanfics from simple oc stuff to incredibly detailed au's with super specific worldbuilding stuff. The same with Undertale and honestly a lot of other series I've gotten into over the years. Sometimes it's fun just "playing dolls" with your favorite characters or your own ocs or both without having to go through figuring out your own unique worldbuilding or finding a way to adapt it into something original that still makes sense.
That said it doesnt mean you don't have folks doing both. I think it just sometimes comes down to how much labor someones personally willing or able to do themselves on something. Sometimes a person wants to just make fanworks and thats fine and sometimes folks wanna make original stuff. Both have value and if you think about it can at times feel like their own seperate cannon from the original works their based on it; just might not be something that goes all the way to pro pub but that's entirely up to them and their own personal desires.
Like not to single them out but modmad is someone i think of who makes incredibly good fanwork, like their recent ongoing Dr. Who fancomic, Into the Inkwell, but also has a badass (and quite popular) original comic, The Property of Hate, so like i mentioned it's not like you cant do both but sometimes you wanna play dolls with your faves (or you're plagued by visions and ideas about stories and scenarios and so you've got to commit it to page or it'll never leave you be xD)
So like yeah maybe sometimes I'll see a fanwork and go "dang i kinda wish this was it's own thing" but also sometimes it's just fine as it is being a piece of fanwork connected to a piece of media that person enjoys and that's enough 
