I mean, there's a lot that future academics are going to have to dive into, isn't there? Not just the evolution of fanfic websites from personal projects to the early one subject archives (oh, I miss slayer.net) to the big modern archives like AO3.
No, what I'm talking about is stuff like sex pollen.
Sex pollen, for those unaware, is a literary device to get characters shagging. That's all. Its a literal handwave to start the sex and it is a legitimate genre of fan fiction. When people just want sci-fi characters to shag in what the tags of AO3 term “porn without plot / plot what plot” they roll out the sex pollen.
Then there's slash fiction as an act of consumer protest. Why are there so many stories of Kirk and Spock getting it on? Geordi and Data? Because for its first fifty years the most liberal franchise in mainstream science fiction didn't have a single LGBT character who was there for anything other than to be a one episode tragedy.
I mean, the whole concept of slash fiction started with a Kirk/Spock fic back in the fanzine days. Now its open to any pairing you can think of (and some you wish no no one had...).
The history of fan fiction is just one big exercise in reader response theory. People consume the official stories and respond to them with their own creative work. This isn't unprecedented, The Lord Of The Flies was actually written in response to another novel (RM Ballantyne's The Coral Island) with a pretty similar plot but with a happy ending in which the boys survive and thrive because they're British and civilised. The only difference now is that it can be done by anyone in response to anything.
There's a character in Steven Universe so far identified only as “Mystery Girl” that Pearl flirts with in one episode. She's appeared once, has no proper name (and is identified only as “S” in a note she writes) and she has no lines. There are a whole bunch of fanfics theorising about her future relationship with Pearl: how S will react when she discovers that aliens are real and how much danger the Earth is in or how she'll react to Pearl's past as a slave. I've read any number of great stories just speculating how a relationship the show might never revisit will go.
Its also a safe and largely anonymous space where people can work out their feelings about sexuality, gender and a number of other personal questions. Whilst it shouldn't be treated as a sole educational resource on the subject, I've read far, far more treatments of non-binary identity, gender transition, gender dysphoria, non-traditional relationship structures (monogamous and otherwise) and even discussions of sexual health in fan fiction than in officially published literature and I really hope that some of these authors break through and we get those sorts of discussion in more mainstream spaces.