Of any A-Listers I would certainly say Lady Gaga. Out of anybody, I think Samantha Barks is the perfect choice. I said it when she sang Defying Gravity on I'd Do Anything, but her voice has grown so much in the last 11 years that I would love to see what she does with the role.
Full Dream Cast:
Elphaba: Samantha Barks, Lady Gaga, Vanessa Hudgens
Glinda: Dove Cameron, Anna Kendrick
Fiyero: Darren Criss, Derek Hough
Boq: Daniel Radcliffe, Darren Criss
Morrible: Meryl Streep (but almost too obvious)
The Wizard: Hugh Jackman, Richard Gere
Dr. Dillamond: Hugh Jackman
Nessa: Anna Kendrick, Vanessa Hudgens