Sally Durant Plummer, I don't see Patti Lupone as Carlotta. I don't really think of Patti as having the operatic soprano voice necessary to play Carlotta. Audra McDonald is more of a mezzo, but I think she'd be terrific as Carlotta. So would Sarah Brightman -- perhaps for the 30th anniversary. 
I would love to see Brian d'Arcy James and Christian Borle team up in "The Odd Couple" or "La Cage Aux Folles."
I would love to see James Barbour as Javert, but he may have played it. He'd also be a superb Emile DeBecque, to Mary Michael Patterson's Nellie Forbush.
I'd like to see Tam Mutu get a shot at the Phantom in POTO, and not just in LND.
Finally, I'd like to see ANYBODY as ANYTHING in Rebecca.
Audrey, the Phantom Phanatic, who nonetheless would rather be Jean Valjean, who knew how to make lemonade out of lemons.