They would be insane to make the novel into a film rather than the show.
The show is what made WICKED popular. Plus, they said it themselves; it didn't work as just a straight film which is why they went down the musical route in the first place.
However, somewhere down the line they should definitely try a dark, true to the book WICKED film...just not now. Tim Burton would be an awesome idea for THAT film, just not the movie musical.
I like the idea of Kristen Bell, but Anne Hathaway doesn't have the voice for Elphaba. There doesn't seem to be much power or belt to it, which is crucial to pull off the big numbers Elphaba needs to sing. She has a sweet voice, which would go better with Nessarose
I'm still all for splitting the character into young + old and having Idina + Kristen playing the older. It's the best of both worlds, we get to see them shine...but also have the opportunity for younger talent (names, by all means) tackle the younger parts.
Oh, and James Marsden for older Fiyero