My top choices would be Megan Mullally during her break from taping WILL & GRACE or Jane Krakowski, who is younger than most of the names being tossed around but who'd bring a Murphy-level of joy and sensuality to the part. More realistically, Reba would be fantastic. And though I imagine the Audra McDonald rumors are just that, she'd make for a glorious Dolly. Gosh, think of what a brilliant Dolly Levi Jan Maxwell would have made (not that I realistically think she'd have been cast as a full-time Dolly).
Let's also hope they get a fine comedian like David Hyde Pierce as a Horace replacement. I adore Victor Garber and thought he was a sweet Horace -- I missed the sharp comedy and no-nonsense approach of David Hyde Pierce nonetheless, it makes the emotional and cathartic payoff from the prelude to "So Long, Dearie" through the end of the show that much more powerful.
"Some people can thrive and bloom living life in a living room, that's perfect for some people of one hundred and five. But I at least gotta try, when I think of all the sights that I gotta see, all the places I gotta play, all the things that I gotta be at"