Acknowledging in advance that this point is somewhat irrelevant insofar as a discussion by us theatre fans of our personal preferences, opinions and predictions for what sort of Fanny Ms. Ambrose might make (wow, that was a long caveat):
I do put some stock in the almost certain fact that the keepers of the Styne, Merrill and Lennart estates have approval over whoever is chosen to play Fanny (just as they likely had approval over Sher as the director) and that they have a strong vested interest in ensuring that the choice is a good one. As she is not a proven musical theatre performer, I'd even be willing to bet that she had to audition for them in some fashion.
Now of course, they are as fallible as any director or casting director or any of us and it's entirely possible they could put their faith in the wrong choice. But if all of the authors estates, and Bart Sher, and the casting director and the producers all believe she is "the one", I'd like to believe at the very least that Ms. Ambrose couldn't possibly be as catastrophic a choice as some on this board have made her out to be. At worst -- speaking just for myself -- she's an intriguing choice and worthy of a "wait and see" attitude.
No one is Babs and no one will be. But I for one am not really interested in seeing someone go that route. I'm much more interested to see if it's in fact possible for anyone to crawl out from under that shadow and make that part their own.
And with that, I lean back and wonder what a BWW message board would have looked like in 1974 when Angela Lansbury was announced as Mama Rose in the revival of "Gypsy".
"No matter how much you want the part, never let 'em see you sweat." -- Old Dry Idea commercial