I want to second the person who suggested Leslie Caron for Solange. It would be criminal if she is up to it and, wants to do it, and does not get the role. Re Lansbury, I don't think she is an ideal fit, but -- if she is up to it -- she could do Broadway Baby (she clearly could belt in the past, and she has always been a GREAT actress, needed to put over the brassiness). Her age would also add a whole other level of pathos.
I am a big Michelle Pfeiffer fan, but she may be too old for the role -- I think of Phyllis as being late 40s; it will probably depend on whether they try to go for a male star (like Jackman) for Ben, i.e., is Phyllis hired first or second? I think Renee Zellweger and Catherine Zeta Jones would be serious candidates for Phyllis if they try for someone with movie name recognition. I also think Zellweger would be a good Sally...she does frailty very well. Maybe we will have a repeat of the Chicago leads.
I do like the idea of Audra for Sally, with a humungous BUT. If the movie takes place in the time-span for the original Follies. a white Ben and a Black Sally would never have been viable, especially given Ben's career drive. I think it is easier to be color blind on stage than on film...I don't think the authors intended the show to be about inter-racial relationships the 40s through 60s.
I would love to see Patti Lupone as Carlotta. I don't think anyone would be as good as her. Carlotta can be any age (I can't remember if she is in Who's That Woman, but she could easily be excluded in order allow her to be older.
It would be great if they could cast a real dancer to lead Who's That Woman; since she was supposed to be in the Follies around the same time as Phyllis and Sally, she would probably need to be in her 50s.