One of my big themes is "There's more to Oz than the Yellow Brick Road." But what do I mean by that?
On the surface, it's simple: While everyone has seen the MGM movie (and its various sequels and re-imaginings) a lot more has been written about Oz than most people are aware of. And it's really fun stuff and I think people are missing out if they haven't read it.
Below that, it gets more interesting: Oz is a land of imagination. It's a place where (to quote Judy Garland) "the dreams that you dare to dream really do come true." I think that because of the success of the classic film, people tend to treat Oz as a "one-story world", meaning that, no matter how well or thoroughly crafted, it was all made to support that one particular story.
Terry Pratchett describes the Discworld as "a place where stories happen." Oz is very much the same. There are all kinds of stories waiting to happen in Oz, only a few of which have already been told. There is always something new to discover and explore.
This is one of the reasons that I think Oz is perfect for adaptation into an RPG. It's possible to tell stories that emerge from the existing lore (like the Tin Woodman tracking down his lost love in The Tin Woodman of Oz) but also to do something that has no real connection to Wizard (like Tik-Tok of Oz). New original characters are a regular occurrence (Patchwork Girl, anyone?).
There really are no limits to an Oz story, and I hope that Adventures in Oz (when it comes out) helps support that.