Someone mentioned writing into a gaming world as fan fiction, which I wonder about.
I've taken gaming campaigns or ideas for them, and fleshed them out into a narrative either for the players benefit or to give myself an idea of what else might need to be in the game for the players to come across to make things interesting. I am thinking specifically of Classic Battletech in this regard, well before Stackpole and FASA brought us the Clans, and how absent a few sourcebooks how bare and wide open the universe appeared to be.
One thing we took to doing was keeping the hell away from storied units, people, and neither I nor my friends ever got up the nerve to send in any of these stories to the Battletech magazine that was run for a while (what was it called, MechForce or BattleForce, something along those lines....). I don't really see that as fan-fiction in the sense of taking established characters and a main storyline and writing around it. Sort of like how some authors try to sell short stories or book treatments to WotC or whomever is responsible for D&D related novels and such.
On the other hand, I have never gone looking for, nor do I have any desire to read, fan fiction related to novels, movies or television series that I have seen. It just doesn't seem right to me, somehow.