I tried recently to write an Oz book called The Great War of Oz but I have enough stuff to work on already so I abandoned it. Here's what I have for the plot so far.
The Nome King was last seen turned into a cactus by Himself the Elf. Naturally, he didn't want to be a cactus for the rest of his immortal life, so he hatched a plan.
The elf had neglected to remove his magic, so he enchants his spines so if anyone touches them, he'd be turned back to normal.
A Winkie happens to walk by and pricks herself on the cactus, restoring the Nome King. He escapes underground to plan revenge.
Here's where it gets good. :)
He deicdes that he's outmatched by Dorothy, Ozma, Tattypoo (Good Witch of the North) Glinda, and the Wizard... so he decides to bring the Wicked Witches of the East and West back to life!
He does this with an ancient spell book, and the Witches' remains are brought to his hiding spot and brought back to life.
The Wicked Witch of the East is confused; she wonders where her silver shoes have gone. The Nome King tells her that they're lost forever in the Impassable Desert.
The Wicked Witch of the West is very angry because her wolves, crows, bees, and Golden Cap that controlled the Winged Monkeys are all gone or used up.
The Nome King tells them that Dorothy and Ozma are the princesses of Oz, and that they should probably go after Ozma first.
The Witch of the West hits him with her umbrella, saying if Dorothy came to rescue her, they can't hurt Dorothy because of Tattypoo's protection kiss.
The Wicked Witch of the East gets a grin on her face... and she says when a Witch dies, her magic does as well. (The shoes were magical on their own, and that's why they still worked)
So they decide to kill Tattypoo, thus removing the magical kiss so they can defeat Dorothy and regain control of Oz.
Meanwhile, Glinda's Magic Book of Records says the following.
"The Nome King has resurrected the Wicked Witch of the East and the Wicked Witch of the West, and they plan to take over Oz."
That's all I have so far.
Anyone else have ideas for Oz books?