Jessica would definitely be able to bring the dramatic intensity to Mama Rose, but she would never be able to sing that score. Is it too early to think about another revival of "A Little Night Music"?...
Several years back she said in an interview that "Long Day's Journey" would be her last Broadway outing. That she was going to slowly retire from acting, but wanted to make sure she played her favorite role on Broadway as her last stage role. Obviously her plans have changed because she is not retired, but there's no guarantee that she will indeed come back to Broadway.
Jessica Lange is one of my all-time favorite actresses. She has said in interviews that she is by no means a singer. I don't think that she would waste her time in musicals where she would overuse or potentially ruin her voice. She could be a decent Madame Morrible in "Wicked", but really any other musical would be a waste. She could potentially play Madame Thernardier in "Les Miserables"...