I love to write it and I love to read it, as long as it’s well written and doesn’t go too far against the character’s grain as officially depicted.
I actually like to create a character to interact with established characters. I try not to be too “Mary Sue” but what can you do? LOL! In my Ares (of Xena & Hercules) fandom heydays, a bunch of us wrote sort of role playing stories and that was fun.
Fanfiction can be an interesting exercise in writing. 🙂