There’s a great interview with Dave Filoni that’s just been posted to IGN. Here are some salient quotes:
Not to say that we would do this right, but I could see a scene far down the road where the Rebel [fleet] amasses near Sullust, and who knows if [the Ghost crew] were there or not? They were just off camera and maybe they got assigned to go do something else. […] Yeah, so there’s some other groups that’s their own story and we’re telling the story of one of them.
This is important, and it’s easy to miss its significance: with Anakin knowing Jabba, Boba Fett being related to the clones, Anakin building Threepio, etcetera, etcetera, the Star Wars universe has been shrinking daily over the last 15 years. What Filoni is saying, essentially, is that we need to explore all those parallel stories that will help to re-expand what has become a contracted universe.
See that’s one of the interesting things about—I guess we’ll call it “new continuity”—is that you’ve read [A New Dawn], so now you’re ahead of the game for people who haven’t read the book.
The old continuity system, with its levels of canon, was prevented from doing this—a film could inform a novel or comic but not vice versa, since the two media existed on different levels of the hierarchy. A novel could never give you key information of the events happening on-screen.
This has now changed. In “Rise of the Old Masters” we see Kanan struggle with his role as a Jedi mentor to Ezra. But why?
So it became this interesting thing and we loved the idea of this young Jedi that was on his way, but the rug gets pulled out from under him before it’s done. Literally his way of life is destroyed and how does he react to that? And we are telling stories obviously in different media that get into that a little bit and how he reacted, which is kind of not be a Jedi at all.
In other words, Kanan’s backstory—the collection of events that shape his character and lead to his behaviour in Rebels—is being told via A New Dawn and the upcoming Kanan comic series. These are not stories being retrofitted to the character by authors taking their best guess but rather stories that informed the writing process on Rebels itself.
In short, then, the Star Wars universe is being rehabilitated by being expanded in a cohesive (not ad hoc) manner. This is the start of something great.
Update: Here’s Filoni specifically discussing “Rise of the Old Masters”.