What makes a text appropriable?

What makes material appropriable? There is a ton of Harry Potter fanfic out there. That makes sense given the sheer numbers of people who read the series. X-Files was huge, but the fanfiction dwindled at a point. Same with Buffy. I've noticed that series in progress are much more compelling than completed series. This is related to Henry Jenkins's observation that people write fanfic because they are both enthusiastic and frustrated. Much of the X-Files fanfic died when in the final episode, Mulder and Scully finally got together. In many ways, it was more interesting for fans to make their own stories about Mulder and Scully than to see how it actually played out. And once it was played out, the fanfic wasn't as compelling because the series had already resolved the issue.

So there has to be a rich world, but gaps in that world where fans can explore and create. It's a context with particular rules-- a starting point. The starting point isn't just having information, though. I've seen RPGs that try to start based on the lives of celebrities like Britney, Justin, etc. These haven't taken off because a certain amount of context is missing. The celebrities also live their own lives, so their daily actions take away a fanfic author's control. The most successful celebrity RPGs I've seen put the celebrities into a particular context, such as if all of them attended a private boarding school or college together. This gives the writers much more freedom because the celebrities have a reason for being in the same space, a certain rule set to follow (school and school type activities) besides freeing these characters from the real people who have their own lives.

So agency is inextricable from this. The fanfic author needs authorial control. When series are completed, it resolves issues that fanfic authors were exploring, taking away their agency to make things happen to characters or between characters, but new books in series provide rich new material and renew interest in fanfic.