Susan Hayward
It bothers me that Fox didn't consider Susan for the lead. She was the top female moneymaker for the studio in the 50s. She starred in two musicals for Fox; I'll Cry Tomorrow and With a Song in My Heart. Both films were huge box office winners and won Oscar nominations for Susan. Susan would have been the right age for Dolly Levi and she's one star whom Walter Mathau would have enjoyed working with.
It's unfair that she wasn't given this chance to do Hello, Dolly! even as Susan did Fox a favor by playing Helen Lawson in Valley of the Dolls, another box office success. She looked stunning in this film and she was sensational when she performed I'll Plant My Tree.
Director Gene Kelly should have taken notice, and imagined Susan in that gold lame gown making her entrance at Harmonia Gardens to sing the iconic title song. After all, a star is a star is a star!