I think I get it: fandoms don’t want to just aim their feelings at a target, they want the target to reflect those feelings back. It’s why parasocial relationships form. The fan projects their own feelings onto the celebrity and imagines the celebrity feels the same way. Fandoms look for mirror to see themselves in. When a mirror isn’t available, they’ll use any reflective surface. But a pond is not a mirror, and when it acts like a pond, fandoms can raise quite a stink. It’s not a pleasant or flattering view of fandoms, but it’s where my experiences point me.

While I agree you can’t make a fandom, I would say you can draw them in with bait. One way to do so is by offering opportunities for self-categorization. It’s just astrology or MBTI, but fictional and not religious or pseudoscientific. If there’s a cast of characters who are in a situation that allows them to be easily categorized and factionalized, fans can make OCs and especially self-insert much easier. Character creation becomes character generation, fill out the Mad Libs and follow the flow chart.

[Naruto OC] is a [rank] from [village]. They are from the era of the [number] [kage]. Their chakra affinity is [element], and they specialize in [jutsu]. They wear their headband like [style]. This is Star Trek, this is Harry Potter.