I agree that other Glinda's seem to be over the top. I love Cheno a lot and think she brought a lot to the role but my favourite Glinda has to be Dianne Pilkington, especially now she's lost the chipmunky voice. I also love Helen Dallimore.. I can't stand Annaleigh though..
My favourite Elphaba has to be Alexia Khadime (current London cast), she's very Idina-like and has a fantastic voice on her. She also plays the part totally different from any other actress I have seen. Kerry Ellis is good, although I think she's a little weak with her acting. I like Idina but she's very hit and miss with her voice. A friend of mine seen Idina in London about10 times and said she only hit the notes in Defying Gravity twice..
I really think they'll cast either unknowns or big Hollywood names like they did with Mamma Mia. Idina and Kristin would be a little bit older when the film eventually gets made and I dont think they could carry off "being teenagers" anymore. Also I think Universal would want the film to be a commercial success and although Idina and Kristin have done some films/tv work, I wouldn't say they're huge stars... meh, we shall see.