I don't think Sherrie Scott can yell that high.
I think the hardest to replace would be Madame Morrible - Carole Shelley couldn't be more perfect!
And, although I doubt Adam Pascal would tour, he'd be a fun Fiyero - I like the rock edge to Norbert's voice and Adam has even more.
And I think the tour will be very successful. Like it or not, Wicked appeals to the masses - it's a new twist on one of the most popular stories of all time, it's got great tunes that are catchy and it's just got a cool title! A lot of my non-theatre friends love it and listen to it all the time. Time will tell!
...and your children start to leave you from the moment they are born.