I can see how LoTR fanfic might run the risk of heresy :) On the other hand I think it could be acceptable fanfic as long as it's written outside of Tolkien's original storyline and leaves the original characters and plot intact. I used to have the old ICE LoTR rpg which had a huge color map of Middle Earth and I liked to envision stories taking place in parts of the map Tolkien didn't use in LoTR. I used to do that with Karen Wynn Fonstad's Atlas of Middle Earth, too--I'd make up D&D modules using her maps. That was fun :)