I generally stay away from fan RPs and fan fiction for the same reasons, with a few exceptions where the world might not be original but is designed to be open enough for new characters and stories to be added anyway.
for instance,
Miskatonic University is set in H. P. Lovecraft's
Cthulhu Mythos. Although Lovecraft was the one who originated it, other authors have since contributed to the mythos and Wikipedia describes it as a "shared universe" for this reason. It was designed to be open and broad enough for anyone to contribute to, as long as they followed the general lore.
similarly I also joined an
Elder Scrolls RP, which if anyone has played any of those games you should know how big and open the
Elder Scrolls world is. It's not a fixed world with a single story, it's a broad and open world where you can easily make your own story. Even more so with the previously mentioned
Vampire: The Masquerade RP, which is set in a roleplaying setting that was built with the explicit purpose of having other people make characters and write stories within it.
working within an existing world gives you a set of rules and
constraints which can be interesting to work with and test your creativity. It also makes it easier on the creator of the RP, as they can set up the whole thing much quicker than if they had to create a whole world from scratch, and this way other people will already be familiar with it.