Some of these are still possibilities:
Nathan Lane or Harvey Fierstein as Tevye (if Fierstein can resist the urge to camp it up; Nathan Lane would be perfect and may have played the role).
Gerard Butler or Jeremy Irons or Patrick Stewart as Henry Higgins. (There’s still time for Gerry, but the other two have gotten too old, despite the current tendency to cast actors in their sixties as Higgins, instead of their forties).
Johnny Depp or Sacha Baron Cohen as Alfred P. Doolittle.
Sierra Boggess as Eliza Doolittle or Queen Guinevere.
Any of the tall, good-looking Raouls (e.g., John Cudia, Sean MacLaughlin, Kyle Barisich, Patrick Wilson) as Freddy Eynsford-Hill, Sir Lancelot, Frank Butler, or Billy Bigelow.
Hugh Jackman or Mandy Patinkin or Colm Wilkinson (twenty years ago) as Emile de Becque.
Julie Andrews (way back when, if she could have mastered the accent) or Kristin Chenoweth as Nellie Forbush.
You alone can make my song take flight.