I know Stephen Schwartz mentioned in March during his birthday celebration that they had a script they were still working on but had already turned it in to the producers and director who liked it. Since it's been a few months now I assume they have finalized the script and are now moving ahead with pre-production by hiring a production designer and a costume designer.
I know someone mentioned earlier the La La Land production designers might be coming on board as they were spotted on the universal lot but IMDB has Dennis Gassner listed as the production designer.
Also it seems Stephen Daldry has not one but two theater productions in the West End this fall, the production designer is also working on Bond 25 which start filming in early December and a film called Greenland which also shoots this fall/winter. It seems unlikely given the scheduling conflicts of several key members of production that the film will be shot this year.
It seems more likely that the film will be shot early 2019 unless Stephen Daldry is directing Obi Wan which apparently is set to begin filming April 2019 and would probably last until October/November.
But anything is possible and just because they've listed the crew only recently doesn't mean they've been recently hired. They could have been on board for months and could be ready to start filming this fall/winter.
Casting is anyone's guess. Stephen Schwartz said back in 2016 at comic con that they want actresses who can sing the score as written. They aren't worried about big names, so it's likely Elphaba and Glinda will both be unknowns, especially as both are very young. Roughly late teens/early 20s when the musical starts and mid/late 20s when it ends. Somewhere between 18-26 is my guess. The supporting cast will probably be well known actors ala Mamma Mia (also made by Universal). Casting for musicals tends to take 3-6 months? Steven Spielberg's West Side Story adaptation started casting in January and had open calls in May and nothing further has been announced so I assume they're in final stages of casting, if it hasn't fallen through. Disney's Aladdin also had an open call last year in March and announced mid July their Jasmine and Aladdin, went into production September 6th and wrapped January 28th. It's set to be released Mary 2019.
Depending on where they film Wicked, a significant portion of the cast could be British like how the new Star Wars films are filmed in the UK and as a result, most of the crew and cast are British. Though seeing as the Wizard of OZ is American at it's core, they probably will have American accents.
Ultimately there is no way of knowing when the film starts production. It seems most likely giving the scheduling conflicts and time it takes to cast a feature film, assemble a crew, etc. It will be this winter by the earliest (late Nov/early Dec), or next fall/winter after Stephen finishes work on Obi Wan (if he takes the job) or early-mid 2020 after Stephen finishes post-production work on Obi Wan (again if he takes the job). My guess is regardless of when they start production, the film probs won't grace our screens until 2020 or 2021.
