My money is on Nicole Parker returning to the role after leaving the first national tour - she and Alli were an excellent pair awhile back, and I'd love to see them back together.
Also, the standby who will be filling in for Donna is not someone whose been mentioned on here yet.
"Anybody that goes to the theater, I think we’re all misfits, so we ended up on stage or in the audience.” --- Patti LuPone.