As far as the ancillary characters, should Heidi and "One More Kiss" make it in to the show I've been saying for a long time that the ultimate coup for that role would be to lure Teresa Stratas out of retirement, or, if that proved impossible, Julia Migenes. Fleming would be fine, but she still reads relatively young (despite being 60), and I feel like Heidi should be one of the oldest of the former Weismann performers.
Melissa McCarthy also reads far, far too young for Hattie, but the idea of Angela Lansbury doing "Broadway Baby" thrills me to no end. And while Chita's French accent in Nine was a bit inconsistent (at least the night I saw it) I now have that image of her as Solange firmly planted in my mind and can think of no others.
I wish Liza was in better health and more reliable to play Carlotta, as that's the closest we'd get to hearing her mother perform "I'm Still Here."
And heck, we might as well just assume that Queen Latifah will be playing Stella, since I don't think you're allowed to have a musical project on film without her these days.