I like Deborah Cox, but her age (despite how good she looks, she still looks like someone who is 25-35) would make her seem obtuse in the film if she was playing one of members of the group. She could perhaps just play another performer of the period, though.
It's fun to speculate on these things, but what we think probably doesn't much matter overall. I do however wonder if the people involved in the casting and producing of the film ever read this board.
"The gods who nurse this universe think little of mortals' cares. They sit in crowds on exclusive clouds and laugh at our love affairs. I might have had a real romance if they'd given me a chance. I loved him, but he didn't love me. I wanted him, but he didn't want me. Then the gods had a spree and indulged in another whim. Now he loves me, but I don't love him." - Cole Porter