It's actually pretty common I think. Star Trek, Doctor Who, and Star Wars all have pretty extensive canons as well, often produced by fans who manage to get their ideas into the real deal. (Doctor Who having a ton of actors who started off fans, two Doctors who are lifelong fans, the two latest showrunners also fans.) Sherlock Holmes is constantly being reinterpreted, mostly via television and film but there are a decent amount of books written using the characters. And they're old, too.
The Star Trek novels were what helped to create my interest in Star Trek. I was always a reader over watching television, so being able to read adventures about the same characters that my father watched on television was really great for me.
There's quite a few examples of what I like to call "published fanfiction." The book series I'm reading now, DragonLance, has over 190 books in the series, written by a group of different people. The whole thing started with a trilogy of books written by two people, and they were able to expand the story to three more books. From there, the series grew to the mess it is today.
The DragonLance trilogy that I'm reading now is written by someone who wrote DragonLance fiction to start. He wrote a short story about a character and sent it in for the creators to read. They liked it so much that they published the short story in a collection, and now he's written five books in the series (that I know of).
For me, I only write fanfiction. The worlds that I write in are places that I want to explore further, or the characters are those that I want to spend more time with. Two of the franchises I'm writing for have their own clichéd plots. Pokemon has the original trainer saving the world idea. Digimon has the rewriting of the second season. But each story that I've read in those franchises about those stories all have their different takes on them because each author is different. The Digimon Adventure 02 fanfic that I'm reading is completely different from the Digimon Adventure 02 fanfic that I'm writing.
In short, there's really no such thing as a completely original story. You can summarize Star Wars and Harry Potter in the exact same way, and yet they're wildly successful franchises that people enjoy.
Writing original works or writing fanfiction presents different challenges for the writer. There's a writer that I follow who once thought that writing fanfiction was completely easy until he tried it. And he said that writing fanfiction was just as challenging as writing original work.