Thank you for posting that themes link! I fell in love with the concept of theme lists in Fullmetal Alchemist fandom, where there's one for just about every popular pairing and a few crack ones as well, so a Yami one is really neat. (I'm just glad there are only twenty; I think I'm going to lose my sanity before I finish the Roy/Ed 45.)

I think that there's no point where one should stop writing a fanfic, but you should probably only expect readers to follow you as long as you can back up your ideas reasonably in the canon itself. I've seen too many works based entirely on fanon, and that's the sort of thing that's really self-indulgent.