I think Hathaway and Witherspoon are good ideas.
I also think that Kate Hudson would be funny as Glinda.
Nadia Bjorlin is another person who I could see as Elphaba. If she gets the role of Wonder Woman (which she's supposed to be in the running for) and it makes her more well known, she might be a major candidate.
"The gods who nurse this universe think little of mortals' cares. They sit in crowds on exclusive clouds and laugh at our love affairs. I might have had a real romance if they'd given me a chance. I loved him, but he didn't love me. I wanted him, but he didn't want me. Then the gods had a spree and indulged in another whim. Now he loves me, but I don't love him." - Cole Porter