"Canonicity" is the perfect metaphor for becoming an adult. Small children learn (to a greater or lesser extent) that the world is made out of strict, absolute rules. They soon come to expect those rules to be both internally and mutually consistent, and the more so they appear to be, the more the child becomes attached to, and identifies with, those rules. As we grow up, and our critical sense develops, many of us start to see that more than one rule is possible, and that some rules don't work in all situations, and some rules from the past are incompatible with rules in the present. And some present rules are incompatible with our experience and observation.
Eventually we realize that what we thought were rules were barely even guidelines. They were intended as simplification - either by us or for us - to give children a means of interacting with a complex, confused and conflicted adult world.
An adult is someone who judges each situation on its present context. Who seeks information, guidance and suggestions from other adults, but who ultimately decides that what is right at any given place and time is what feels right within the present context. It is a sign of immaturity to seek to fit a present context into its place in a set of established rules.
Fiction, in the obvious cases of fairytales, fantasy and (much) science fiction, but in almost every genre, is about rules. Vampires can't go out during the day, police officers must follow procedure, cowboys are bound to being the good guy or the bad guy, and can't go against this even if it is in their nature to do so*. Pterry calls these rules "narrative causality" and with good reason. We teach the rules of our culture to eachother by telling stories.
But... the oldest stories we know are about heroes. And heroes break rules. Orpheus goes into the underworld to bring Eurydice back from the dead (in Zena people go back and forth so much to the underworld that Cerberus is put out to stud and Hades fits a revolving door (at least, I assume so)); Perseus kills anything and everything it's forbidden to kill - Heracles succeeds at every task which has been set him, even though he has been set these tasks by a god who intends him to fail.
Stories about heroism are indeed stories about becoming an adult; taking responsibility, having the courage of your convictions (a favourite expression of my prep-school Headmaster Alan Butterworth) - what I love about this idea is that it suggests that even if your convictions may be wrong, you should still do what you feel is the right thing.
Argument over what is canon and what is not are particularly impossible in Dr Who. Much fun is being had by the series' present creators at the moment to stimulate a controversy about how many regenerations a timelord can have. The series is 50 years old. Hundreds of people have contributed to the Dr Who universe over those years, and they can't all agree though some have tried — heroically — to create stories that reconcile apparently conflicting rules.
Personally, I think there is more meaning to be found in looking at the way that the rules of a fantasy universe, or a folk tale, evolve with each retelling. As an author, there is a huge amount to be learned from reading The Hobbit first, then LOTR, then the Silmarillion. The major cultural flaw of the new Hobbit films is exactly what its makers all to obviously think is its strength: that the audience already knows what happens later, what the real meaning of the Ring and the Necromancer are.
But of course, when the Hobbit was written, the ring was just a magic ring that made you invisible, and the Necromancer just a nebulous and distant threat - something to be avoided, and no more.
Bilbo's ring is retconned into The One Ring; Gollum is retconned into Sméagol; the Necromancer is retconned into Sauron.
Inspiration is essential to the storyteller's art; and just as you can be inspired to write your own story by someone else's, so you can be inspired to write a new story by one of your own stories.
Damon Courtney is very close to releasing the third in his Dragon Bond series, and the third book is a culmination of a process of ongoing development - an expanding fantasy world; an expanding canon. A hero of the third book was little more than a mook in the first and a pawn in the second. But Damon was inspired by what the character was becoming.
To a considerable extent, what we add to the world of the series in later books can be seen as what we did not know. When we read The Hobbit as children, we thought Bilbo's ring was just a magical trinket, and very convenient considering his, ahem, profession. But when we read LOTR, we can choose to accept that in reality there were all sorts of things we did not know; could not know. LOTR shows us that no story exists in a vacuum; no story is an island — what contributes to a story may have causes and consequences far outside that story.
Which is what we learn about real life, when we become adults; the real world does not have convenient beginnings and endings. The real world is all middles.
* the most recent example of this is Vimes, who believes himself to be a bad guy, but is incapable of resisting the urge to do the right thing.