After revisiting the recount of my masters thesis, I realized that it is rather German. That is to say, it goes on at length to establish some general principle, but then doesn't bother to give examples of how this principle is realized. Which is a style of writing well suited for some purposes, but, let's face it, is also rather annoying. So let's contextualize this general principle for a spell, by relating fan fiction to the subject of history.
The general principle is that people learn by doing things that they are interested in doing. This happens automatically, without the addition of directed conscious effort. When someone does something, the doing of that thing places them in situations and frames of mind which facilitate the process of learning, and the more doing that takes place, the more learning subsequently follows. Being interested bring with it the propensity of doing more of it, and of paying attention whilst doing it. It is, in most cases, a self-perpetuating process.
This is rather straightforward, and the biggest drawback with this line of thinking is that it takes too many words to convey with regards to how straightforward it is. You begin reading, work through the verbiage, and then conclude at the end that it would have been sufficient to just say "you learn by doing". Which is true, but it also goes to show how much effort you have to put in to convey something straightforward. In retrospect, it is obvious, but you have to go through the process before it becomes retrospectively obvious.
Thus, we have what we need to get to work: the general principle of learning by doing, and the notion of retroactive obviousness. Let's move on to fan fiction and the subject of history. Specifically, let's move on to how the notion of 'canon' relates to the teaching of history.
Canon, in the context of fan fiction, denotes a particular set of works which can be considered official or true (as far as fictional depictions are true). In the case of, say, Harry Potter, the books written by Rowling are canonical, and the specific words found within these books carry significance in that they are the source material from which all knowledge of the fictional universe are garnered. Any further discussion about the Harry Potter universe will have to take these books as written, and conform to the limits imposed by Rowling having written them in a specific way instead of another.
Or, to put it another way: it is canonical that Harry Potter is a wizard that attended Hogwarts, a school for magically proficient youngsters. It is, however, not canon that Harry at a young age underwent a series of radical medical procedures which replaced everything but his visible exterior with cybernetic machinery, and that he is a robot that passes for a human child. The former is canon, the latter I just made up. Those who want to talk about what happened in the narrative universe of Harry Potter have to stick to what actually happened in the narrative - which is to say, the source material, as written.
Any particular work of fan fiction set in a particular narrative universe has to be related to the source material, in various ways. The fan work has to cohere with the source material (i.e. be about wizard Harry rather than cyborg Harry), and it has to cohere enough that assumptions from/about the source material carry over to the fan work. The closer to the source material a fan work manages to cohere, the more interesting things it has to say about the canonical narrative universe.
This introduces an element of evaluation to the act of reading fan fiction (and even more so to writing it). The act of reading also becomes an act of comparing - does the fan work cohere with the source material, and if there are inconsistencies, where are they? A critical reader can move back and forth between the different texts to find out whether they cohere, contradict or - more interestingly - pose further questions about the source material that are revealed through the act of writing the particular work in question.
Whether or not a reader actually makes the effort to make such comparisons depends entirely upon their level of interest. But, as we stated at the top of this post, people do the things they are interested in, and it is by doing the things they are interested in that they end up learning what they actually learn.
Thus, those who are interested in fan fiction about Harry Potter will eventually learn the skills associated with comparing a fan work with canonical works, by virtue of following their interest. They will find out which works are considered canonical, which works are not canonical and which works occupy ambiguous gray areas between these two poles. Or how to handle situations where canonical works disagree - such as when the books and their movie adaptations contradict each other. Which canonical authority has preference?
If you are a teacher of history, then these are the very questions you wish your students to engage with. Not about Harry Potter, mind, but about the general validity of narratives told about the past. Which works are canonical, which are not, and what do you do with all the gray sources in between? Which statements about the past can be substantiated with references to the source material, and which are but speculation? How do you position yourself as a critical reader with regards to the source material at hand? What do you do when you encounter a text about a historical equivalent of cyborg Harry? These are questions that practitioners of fan fiction engage with, albeit not always explicitly.
The pedagogical challenge that follows from the general principle that learning follows from doing what you are interested in, is to identify what students are interested in and which skill sets they have developed during the course of following their interests. By doing this, a teacher can utilize the retroactive obviousness inherent in applying what a student already knows to new situations. Rather than restarting from square one, we do something more interesting.
Fortunately, everyone is interested in something. But that goes without saying.