This is a comprehensive list of fanime titles that has at least one episode one. If you have trouble choosing a fanime, there is, of course, the Fanime Realm official: Fanime Recommendation Randomizer! For Fanime that are unreleased or lost media, you can go to a page dedicated for that.
(Please Note that I know I haven't seen all of the fanime here. I am only one person and my brain can easlier turn into scrambled eggs is I think about it too long. There is a reason why there is a submission page.)