ETA: That's part of what I love about the Star Wars, and Star Trek novel lines, they have expanded beyond just the characters from the TV shows/movies.
There are other tie-ins that have done that.
Doctor Who had the Bernice Summerfield novels (spinning off a book-original companion) and various audio spinoffs or side stories, not to mention comic-strip stories exploring a variety of side characters such as Abslom Daak, Dalek-Killer.
Babylon 5's Del Rey tie-ins did a lot to expand the universe beyond the core setting and format with things like the Psi Corps Trilogy and the Technomage Trilogy.
And there have been some tie-in lines that have explored side characters and expanded the universe partly because they were limited in their ability to do anything with the main cast. A lot of the
V tie-in novels introduced original characters resisting the Visitors in various parts of the world (well, mainly the US), most notably the New York resistance in the Howard Weinstein novels (and admittedly not many of the others were any good). And the black-and-white comic books that tied into
Alien Nation all featured original characters and situations in the universe of the show. (I'm not sure they even had the rights to use any of the TV/film characters, since they avoided them so completely. But that doesn't make a lot of sense -- how could they have the rights to everything about the series premise/format except the characters?)