My editor has pestered me to write blogs about how I wrote Outsiders from Oz. My intent is to wait for this until the book is available. I hope that will be soon, but until then, I'll tease you with a few things I learned not to do.
Unless your story requires it, a lot of exposition is not encouraged. In the first draft of Outsiders, I spent many paragraphs talking about Jellia Jamb. None of what I wrote was bad or anything, but it was all eliminated in the final version. In fact, I'd rewritten the first chapter from scratch.
The reason why was because Jellia only appears briefly in the book. This was always the intention for the story, so why spend so much time on a character who isn't in the book for very long? Jellia does get a brief introduction, but that's the key: it's brief.
Exposition should be kept on a strictly need-to-know basis. Of course, I also advise against breaking the momentum of a scene for exposition. I set up a little mystery for the characters in Outsiders, though I'm very sure my readers will piece it together quickly since they get both sides of the story. By the time the story came to the conclusion, everything was ready for a fast paced finale.
Don't drag your story out
While you want your story to fill a good number of pages, if you find yourself slipping in visits to Buxleyburg or Whoozywoozyville for no purpose than to make the story longer, it might be better to reconsider how long you need your story to be. I had intended for Outsiders to be 20 chapters, like most of Thompson's books, but wound up being 16.
If you find yourself adding in filler material, try to rework it to work with the plot. Since Outsiders follows two separate parties for a large portion of the book, I found myself needing to fill some time between when the two parties would meet up. So, I came up with an additional episode that would introduce a new little creature to Oz's zoology. However, I wound up making them part of the story's conclusion.
And that was much better than the original version of that chapter where they literally sat and talked around a campfire. That was just too tonally different from the rest of the story.
Don't throw in everyone
We all have favorite Oz characters, and when you write an Oz story, it's almost a no-brainer to grab from that list of characters. However, this can easily become a difficulty: once you have a character in, what are they going to do?
There is one character in Outsiders that I wonder if people will mention when they review or comment on the book. He is one of the main characters and one of the most-loved, but even I had to look at him at the end and realize he didn't do much. However, considering what I'd already written, I could neither drop him or expand his role in ways that wouldn't feel out-of-character or absurd.
One Baum case of too many characters I remember quite well is in Glinda of Oz. Many, many characters go to the island of the Skeezers to rescue Dorothy and Ozma, but it could easily have been pared down to just the Wizard, Glinda, the Scarecrow and Scraps. Another case is having all three formerly American girls in Dorothy's search party in The Lost Princess of Oz. While this was Baum's intention to have an adventure with these three and Scraps, the girls don't have much of a chance to do much with all of their companions and Dorothy's magic belt.
So, yes. Consider your plot and which characters work best for your story. Also, remember that a common occurrence in Oz books is new characters. In addition to keeping the old characters in character (and build on, if that's what you're going for), you now have new ones to develop. Less characters allows you more room to have your characters grow. That's not to say you can't have cameos, but these should also be worked in relevant ways to the plot.
In Outsiders, I chose against creating many new characters, considering the number of Baum characters I was already using. There are a few new ones, but I spent more time focusing on developing the characters established by Baum, some of them not having appeared in new adventures in over a century. At the end of the book, there are a few cameos, including Betsy and Trot, and a couple other who I don't say who they are. (All in good time.)