At the sake of being called boring, I think Kate Baldwin would make a lovely Marian, a role she has played before. She's of an age to match Hugh Jackman yet also seems young enough to have a brother of ten years old. (Unless you subscribe to the theory that Marian is actually Winthrop's mother and that they have been living a lie and maybe the ladies of River City are onto something when they label her a hussy?)
There's also the option of getting someone even starrier like Audra. She is an ideal Marian and would sound gorgeous.
And David Hyde Pierce would make a splendid Mayor Shinn, alongside someone like Kristine Nielsen or Judy Kaye or Ana Gasteyer as Eulalie McKechnie Shinn.
Are there any character dancers to play Marcellus? I always like it when he dances a lot in Shipoopi.
And they should think of getting a major player to play Charlie Cowell and who could conceivably replace Jackman as Harold Hill the way it was done back in the '50's. Someone like Will Chase or Andy Karl. Would either of them stoop so low to be the alternate? I know they both could play (and Will Chase has played) Harold themselves, but neither will draw in a crowd in the Jackman manner.