The Design Inspiration collected depictions of Dorothy and her companions along the Yellow Brick Road in The Wonderful Wizard of Oz from twenty-three different illustrators. They appear to be influenced largely by:
Above, for example, is the work of Sebastian Giacobino.
The article is titled “25 Various Styles of The Wizard of Oz Illustrations," but there are two samples apiece from Skottie Young and Robb Mommaerts.
For the missing two styles to seek inspiration from, therefore, I propose those of original Oz artists W. W. Denslow and John R. Neill, despite their lack of deviantART portfolios.
(In other Oz news, the play Say Goodbye, Toto opened without fatalities in Los Angeles. Haven’t seen reviews on the web yet.)