Some of these may seem old for the first act, but the characters really need a mature look for the second act. Also there's makeup and visual effects, just look at what the did to Streep in ITW.
Fiyero - Chris Pine. If his "Dancing Through Life" is half as good as his "Agony", I'm sold. Also I think Fiyero should look older (like someone who's been in college partying for a few years.
The Wizard - John Lithgow. A veteran actor, Tony winner, Emmy winner, Oscar nominee who can handle comedy and drama.
Madame Morrible - Glenn Close. MAKE. THIS. HAPPEN.
Boq - Daniel Radcliffe. That's just perfect casting. He can play dorky and already kinda looks like a Munchkin
Nessarose - Phillipa Soo. Just thinking about her singing "Wicked Witch of the East" sends shivers down my spine.
Doctor Dillamond - John C. Reilly. He could be hilarious and heartbreaking at the same time.
Elphaba's parents and the Midwife in the initial flashback - Idina Menzel, Norbert Leo Butz and Kristin Chenoweth. C'mon, this one is was easy, very much doable and I'm sure they'd be up for it.
G(a)linda - Kristen Bell. Her Frozen live performances convinced me, she has the comedic chops for sure and even though she's 36, she doesn't look a day past 30.
Elphaba - Cynthia Erivo. She's making her way into Hollywood, she'll be in Steve McQueen's (12 Years a Slave) next feature, opposite Viola Davis and is starring in a new Harriet Tubman biopic. She's the perfect age and having a black performer, even if covered in green makeup, takes the message to a whole new level. Also, this: https://www.youtube.com/watch?v=u8FeOX2IcO4
A cast like this, with true talent, a mix between relatively well known Broadway performers like Erivo and Soo, Hollywod veterans like Riley, Close and Lithgow. Bell, who's an established TV actress and Pine and Radcliffe, well known to general public with their franchises, is the best option. And everyone on this list is a realistic option with proved singing abilities.