I must note that this blog—discussing writing further Oz stories—is based solely on my own experience writing a few Oz stories. Other writers and authors have likely had different experiences and view points. They are welcome to mention their own through the comments or their own mediums.There's an old saying "Scratch an Oz fan, you get an Oz book." This is very accurate as most if not all Oz fans have come up with new stories about the Land of Oz.
But when someone creates a new short story or book about the Land of Oz or its characters, they have to figure out what rules to set for how they approach Baum's beloved world.
Baum rarely set up rules for how his world worked. In the first book, it became clear that animals could talk and just about anything could live in the Land of Oz. In The Emerald City of Oz, Baum establishes that no one dies in Oz and they have a communal economy. It wasn't until The Tin Woodman of Oz that Baum gave us a bit of backstory about the country's origin.
Thus, there is quite a lot of room open for interpretation.
Later books in the Famous Forty generally create new adventures, but on occasion, Ruth Plumly Thompson and Jack Snow either set a few new rules or attempted to clarify some backstory. However, as Thompson's work is mostly still protected by copyright, the modern writer of Oz stories generally ignores it so as to avoid any legal issues or decides that it hardly matters if Thompson's work doesn't quite match up with their own. (This is not to say that Thompson's work has been ignored: writers such as Paul Dana, Chris Dulabone, Nathan DeHoff and Marcus Mebes have liberally used characters appearing in Thompson's public domain books.)
In sorting out what is and isn't important to them, the issue of "canon" arises in the writer's mind. What, from the Oz books, is definitely history? Early on, it became clear to me that there was no reason why Little Wizard Stories of Oz and Baum's Oz-related fantasies (Queen Zixi of Ix, The Sea Fairies, Sky Island, The Life and Adventures of Santa Claus, Dot and Tot of Merryland, etc.) should not be considered canonical alongside Baum's fourteen novels. The messier issue is Queer Visitors from the Marvelous Land of Oz and The Woggle-Bug Book. While these visits from Oz to America could have happened, Dorothy meeting her friends on her farm in Kansas and then Aunt Em and Uncle Henry being skeptical of her tales of Oz in The Emerald City of Oz proves problematic (not to mention that the first Queer Visitors story dates the entire series). Thus, although the writer might enjoy these tales and even take concepts from them (the Woggle-Bug's four arms and wings), they may decide not to count these canonical.
Recently, though, I have decided that there is no reason why all of Baum's works of fiction—fantasy, non-fantasy, pseudonymous, anonymous—could not all take place in the same world. However, unless there's a good excuse for a crossover, it's not really going to make much of a difference.
Sometimes, you might run into a continuity inconsistency. But don't let that get in the way of a good story! Maybe you can sort it or just decide which version you want to go with.
As seen in some of my blogs, I came up with a timeline for Baum's first several Oz books. The reign of the Wizard in my canon wound up being a brief ten years (versus the 20 or more other writers sometimes use). This was that if people in Oz were aging during the Wizard's reign, why was Ozma still rather young? Dorothy's first four visits to Oz occur within about three years, with her first trip to Oz being when she was eight and her moving to Oz at age eleven.
The writer also has to decide other elements: in The Chronicles of Narnia, trips to Narnia take up no time in "our world," but this is never inferred in the Oz books. One could interpret that Oz time runs longer or faster than the Great Outside World, but Baum's concept seems to be that the two run at the same time. (This is made clear when Ozma promises to look in on Dorothy at a certain time on a certain day, though one could say that Ozma is keeping track of Dorothy's time.) To me, anything other than what Baum indicated unnecessarily complicates Oz lore.
Another point is where is Oz located? Like Narnia, some like to have it so that it's in an entirely different world somehow unreachable by us except through rare, magical occurrences. Others say it's on another plane of existence or an alternate reality. I, myself, go with Baum's concept of a hidden country, likely in the South Pacific. (How people get there, I addressed in an earlier blog.) While this is my idea, I find it to be less cumbersome if this idea is not addressed in the text, though making up your mind about it may help in creating Oz stories.
Finally, there's backstories for the characters. Baum wasn't big on character development, but rather defined his characters. (There are some exceptions.)This has proved a boon for Oz writers as since the characters generally stay the same, they can set their story whenever they wish. However, it may be helpful to decide the backstory for certain Oz characters. Fortunately, Baum tells us the stories of the Wizard, the Scarecrow, the Tin Woodman and Ozma. However, when deciding further details about these and other characters, that is when your imagination comes in. I'd figured out a backstory for the Cowardly Lion that I hadn't planned to write, but I eventually did anyway, and it became the award-winning "The Way of A Lion," now available in Oziana 2013. So sometimes figuring out the backstory of a character isn't just deciding your approach to a character, it can also sometimes lead to a good story!
All I'm saying here is that when you set out to write an Oz story, it's very helpful to have a complete vision of Oz in mind.