As an undergrad, I double majored in English and philosophy. While I ended up swerving firmly towards philosophy by the end (I discovered that I enjoy reading books way more than I enjoy writing about them, and ended up doing only the bare minimum to get the English degree, as opposed to philosophy where I took more than double the required number of credits), philosophy of fiction was a natural place for my two sets of interests to overlap. I went into grad school planning to do something on possible worlds and fictional discourse, before logic captured me and took me away for more than a decade. One of the things I love about teaching undergrad courses is that it has allowed me to get back to those interests and rekindle them.
Unfortunately, if you look at philosophy of fiction rather cursorily (and from a distinctly philosophy of language view, which is the route I come into it from), it's hard to tell how it differs much from the general problem of the semantics of non-denoting words/names/phrases or the problem of nonexistent objects. There seems to be a disproportionate amount of time and pages spent on these topics, and I've become increasingly convinced of the utility of rooting out aspects of fiction that have been overlooked or ignored. In particular, I think there is a lot to be gained at looking at the practices of fiction, by which I mean the production and consumption of it. How do people interact with fiction(s)? Can our philosophical theories, whether specifically aimed at fiction or whether more general, explain what underpins these practices? If that is too much to ask, are these theories at least consistent with our practices?
Now, I don't want to make it seem like no one else has asked any of these questions. One good example of an area of philosophy of fiction that takes seriously our practices w.r.t. fiction is the paradox of fictional emotion.  What I am saying is I want more! More like this, more things that go beyond discussion of Santa Claus and Sherlock Holmes, Pegasus and phlogiston. Don't ask merely "how can we say meaningful things about fictional objects?", go one step further and ask "how can we say meaningful things in fictional languages?"  Don't ask merely "How does the operator 'truth in the story' work?" but rather "How does the operator "true in the story" work when there is more than one story?" And that brings me to an aspect of philosophical practice which has to date been almost wholly ignored by philosophers: fanfiction. 
So, what is fanfiction, and why should philosophers care about it? As Thomas defines it:
The term fanfiction (sometimes abbreviated as fanfic) refers to stories produced by fans based on plot lines and characters from either a single source text or else a 'canon' of works; these fan-created narratives often take the pre-existing storyworld in a new, sometimes bizarre, direction. 
The largest repository of text-based fanfic is available online at http://archiveofourown.org, and when people refer to 'fanfic' it is usually in reference to this sort of production. However, as we'll see below, some of the interesting philosophical questions surround precisely what counts as fanfic and how it should be defined.
But let's take this definition and start picking out topics that are of philosophical interest in connection with it. Whenever my students are struggling to come up with good paper or thesis topics, my advice to them is to pick something interesting, whether it is philosophical or not, and (a) see how it intersects with a particular philosophical theory or topic or (b) start asking the big questions (What are the building blocks at play? How do we have epistemological access to them? How can we speak meaningfully about them? How can we reason about them? What are the grounds on which we can discriminate one answer to these questions from the others?). When you do this with fanfic, you get a plethora of interesting questions. I recently wrote about some of them, namely the nature of the relationship between the source text and the fanfic, and what the relationship to the stories therein is, whether fanfic should be seen as derivative or constitutive. <- This post contains rather nascent ideas, which I have since developed into a more fully-fledged investigation  which was directed at question of type (a): How do two different possible worlds accounts of fiction, Lewis's and Kripke's, account for or explain fanfiction? During writing, I found myself continually coming up with questions I wanted to address but couldn't within a limit of 6500 words. The first group stem around how to define fanfic in the first place:
- How are we to define fanfic? Should it be defined so that it only encompasses text-based works, or should it include things like comic books and films?
- What is the relationship between fanfic and fandom more generally?
- Is vidding ("an art in which clips from television shows and movies are set to music to make an argument or tell a story" ) fanfic? Is cosplay? (On the topic of cosplay, what are the cosplay contexts in which counterfactual accounts can be expanatory?)
- Where does LARPing fit into this?
- Historical fanfic: Is Paradise Lost Bible fanfic? What about the various Irish mythological tales?
A central notion in discussions of fanfic is the notion of 'canon', a standard of measure against which the newly created stories are measured; the existence of canon is "particularly important for the creators of fan texts because they are judged on how well they stick to or depart from canon" , and this brings with it a plethora of questions too:
- How much canon can you violate and still tell a legitimate story?
- How does something move from noncanon into canon?
- Can something go the other way?
- Who has the authority to say what is canon and what is not?
- Where does that authority come from?
- What is fanon, and what is its relationship to canon?
Many people writing on the sociological and anthropological aspects of fanfic stress the transformative, and often subversive, nature of fanfic, which are closely tied to the questions of canon and authority. Without authority, there is nothing to subvert; without canon, there is no way to say that one story is a transformation of another. And this leads us to questions that focus on the created stories and characters themselves:
- How do we identify characters across fictions? Is the Sherlock Holmes of Conan Doyle the same as Benedict Cumberbatch's Sherlock? (These are questions already discussed in philosophy under the heading of 'intensional identity') How about Adam Smith's Batman and Christian Bale's?
- How does "interfictional carry-over"  work?
- In what are The Taming of the Shrew and "10 Things I Hate About You" the same story? Is the latter fanfic of the former? Or is it just an adaptation? Are adaptation and transformation different things?
- How do we understand the metaphysics of cross-over fiction (which takes characters from one story and transplants them into another)?
Some people reading these questions might object that I've betrayed my principles and swerved from philosophy back into English, and that these are questions literary theorists, rather than philosophers, should be answering. I disagree. I think that many of these questions can probably benefit from engaging with the literary, sociological, and anthropological work that's been done on fanfic already, but that philosophers bring with them a special way of asking and answering questions that, when applied to fanfic, can provide material both of interest to other philosophers and of relevance and interest to not only people in other academic disciplines but outside academia altogether. (Most of my FB friends who asked to read my draft  were writers (both of fanfic and of other work) or readers (of fanfic or sci/fantasy more generally), rather than philosophers. Who wouldn't want to write philosophically robust material that is actually of interest to the general public? Besides, writing on these topics means I get to watch and rewatch all the versions of Pride and Prejudice and call it research!)
This post barely scratches the surface, but I hope I've whetted at least a few appetites, and would love to see some work on these questions forthcoming in coming years!
 Cf. Colin Radford, “How Can We Be Moved by the Fate of Anna Karenina?” Proceedings of the Aristotelian Society 49 (1975): 67-80; Kendall L. Walton, “Fearing Fictions”, Journal of Philosophy, 75, no. 1 (1978): 5-27. Also, one of my students, Andrew Thomas, is writing his master's thesis on a fictional realist solution to the paradox.
 Sara L. Uckelman and Phoebe Chan, "Against Truth-Conditional Theories of Meaning: Three Lessons from the Language(s) of Fiction", Res Philosophica 93, no. 2 (2016): 1-19.
 At least, as far as I have been able to find. The only discussion I've found is Roy T. Cook, "Canonicity and Normativity in Massive, Serialized, Collaborative Fiction", Journal of Aesthetics and Art Criticism 71, no. 3 (2013): 271-276. If you know of any other philosophical work that discusses any aspect of fanfiction, please leave a comment!
 Bronwen Thomas, "What is Fanfiction and Why are People Saying Such Nice Things About It?", Story Worlds: A Journal of Narrative Studies 3 (2011), p. 1; cf. Rebecca W. Black, "Fanfiction Writing and the Construction of Space", E-Learning 4, no. 4 (2007), p. 385.
 Sara L. Uckelman, "Fanfiction, Canon, and Possible Worlds", in preparation. Email or comment if you'd like to read a draft.
 Francesca Coppa, "A Fannish Taxonomy of Hotness", Cinema Journal 48, no. 4 (2009), p. 108.
 Kristina Busse and Karen Hellekson, eds. Fan Fiction and Fan Communities in the Age of the Internet, (Jefferson, NC: McFarland, 2006), p. 10.
 David Lewis, "Truth in Fiction", American Philosophical Quarterly 15, no. 1 (1978), p. 45.