Chenoweth and Menzel are both contractually tied to the project if it ever happens. They'd have to be bought out (which I imagine is not impossible, but would be pricey.)
I think the RENT film will actually be fairly influential in the "musicals casting game"--if it does well. They might just decide to start casting OBCs if it becomes huge.
At any rate, WICKED is not going to happen for a LONG time. The one you guys might want to think about is URINETOWN, which DOES have a first draft of a film script, and is being currently being worked on.
And for what it's worth (not much) I thought Rossum was atrocious in Phantom. Her singing was ok, but she seems to have only one facial expression.