It would be a good marketing move, but ultimately I doubt they'd have the nerve to risk screwing up the films credibility with such stunt casting. They will want a blockbuster and awards.
Lea Michelle has long been connected to the film and I think at this point people have just accepted that it'll happen. At least she'll be able to sing the part, and that's going to be a huge issue with stunt casting.
Personally, I think a good balance of stunt and appropriate casting would be:
Elphaba: Lea Michelle/unknown
Glinda: Kristen Bell
Fiyero: Zac Efron
The Wizard: Robin Williams/Tom Hanks
Madam Morrible: Meryl Streep
Boq: Jonathon Groff
Nessarose: Anna Kendrick
Doctor Dillamond: Morgan Freeman
Cameos...
Elphaba's mother: Idina Menzel
Nurse: Kristen Chenoweth
This is totally out there...but it would be interesting to see Gaga play Elphaba. When you think about it, it's both mad casting and yet very appropriate.
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In terms of director, I reckon Baz Luhrman (Moulin Rouge, Gatsby) could do something great with it. Capture the magical world of Oz but not in a non-Disney/Tim Burton way, and inject fresh modernity into the musical to capture the publics imagination. Make it 'cool', but faithful to the show.
Lea is perfect for this role. Her role on Glee support to be super dramatic, she is not overacting.
Hopefully Chris Colfer get a role on this project too. Their duet Definning Gravity and For good is amazing
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