"I have to think you would be disappointed, she doesn't have the vocal chops for it. I admit I loved her Witch in In to the Woods, but Margaret Johnson requires much more vocally."
I almost typed a similar response. Of all roles that should not (and probably never would be) attempted by someone without a major singing voice, Margaret Johnson is right at the top of the list. I suppose she could do, or could have done, a movie of PIAZZA, but (1) there is SO MUCH singing in the role that a dubbed performance would be pretty pointless, and (2) Marni Nixon is no longer with us.
While we're discussing roles that Streep never got around to playing but in which she would almost surely have been brilliant, I'd have to say that Blanche in STREETCAR is probably #1. About 25-30 years ago, to my mind, she would have been perfect for the part in every way, and I can only imagine how great that performance would have been.