I think Mara Wilson said when they announced the film years ago that she’d be happy to have a cameo, which I could see.
I seem to recall reading some people saying Sacha Baron Cohen could play Mr. Wormwood which I’d like to see. He’s a good enough singer, obviously funny, and has enough dramatic talents to pull off the ending. Not sure about Mrs. Wormwood though. Maybe Emily Blunt or Amy Adams playing against type, they both have great comedic chops and strong voices. I don’t know if either one of them are good dancers though.
“Somebody stop me before I sing again” - Bazzard