All of the things Megan Hilty did at Carnegie Melon were belty roles, like Queenie in "The Wild Party." As others have said, she always thought she'd go in for Elphaba, but ended up getting Glinda. Where as I think she may be able to sing Elphaba, she has too much of a 'baby face' look to be right for it.
While I found Kendra Kassebaum to be quite the comedic genius and show stealer when I saw her as Glinda, I think she may be the one person who could actually
play Elphaba. As has been noted with her struggles with her soprano rage, Kendra is first and foremost a belter (something she snuck into Glinda during moments like her belted oppertuny in "Thank Goodness.") She has the quirky look about her to be Elphaba, and if her top notes can be belted, I could see her pulling it off.
Will one person ever play both roles, however? No. They aren't going to implement a track where one girl has to learn two roles of such magnitude.
Tonya Pinkins: Then we had a "Lot's Wife" last June that was my personal favorite. I'm still trying to get them to let me sing it at some performance where we get to sing an excerpt that's gone.
Tony Kushner: You can sing it at my funeral.