I've definitely read somewhere before that the most fertile fandoms tend to be for works that are good enough to make someone enthusiastic about it (whether it be through a cool concept, interesting characters, etc.) but also has flaws that make you want to tinker around with it. The few fandoms that have actually inspired me to write fics for definitely fall into this bucket. The only exceptions I can think of is something like White Christmas, where I have some minor quibbles but overall think it's brilliant, but it's so short that I'm dying for more content about these characters.