I won't comment more on Mazzie since really, I feel every Broadway semi-name/name has been mentioned for the role at one point or another on this thread by people who are "in the know." Plus, everyone seems pretty set on Waddingham.
Marius, I guess your comment is more or less directed to me since I have been pretty outspoken (to an exaggerated level) about how uninterested I am in a Waddingham transfer. The only reason is that we haven't seen NIGHT MUSIC on Broadway in decades, all kinds of names have been attached to proposed revivals at one point or another, and there are so so so many American/British actresses known to the public that I'd love to see play the part. Even just Broadway names who have paid their dues on Broadway and should be given the chance to play this mother of a role. And I just feel for Equity to approve the transfer of a British no-name actress to play a role that everyone and her mother would love to play, makes no sense to me. It's nothing really against Waddingham whom I'm sure is great, will be great, and kudos to her for being brought over.
"Some people can thrive and bloom living life in a living room, that's perfect for some people of one hundred and five. But I at least gotta try, when I think of all the sights that I gotta see, all the places I gotta play, all the things that I gotta be at"