For those of you unfamiliar with Something Wicked This Way Comes, it is the story of two friends, Jim Nightshade and Will Halloway, who were born within minutes of each other on the night before Halloween. Their friendship is tested when a traveling carnival run by the mysterious duo Cooger and Dark comes to town. The carnival (true to its trope) brings the citizens of the town face to face with their deepest desires and fears, usually with messy results (such as the traveling salesman who is made one of the carnival's attractions). The biggest temptation facing Will and Jim is the carousel that will make anyone who rides it age. Jim, the younger of the two, wishes to be older for once in his life, even if it breaks his friendship with Will. Will is content to be as he is, having seen the effects of age on his older-than-average father. They also have to deal with the darker effects of the carnival's presence in the town when Mr. Dark decides that they have seen too much and starts combing the town for them.
As far as the script for the musical goes, Disney's film (scripted by Bradbury himself) would be a good starting point. The script covers the plot and themes of the book very well and, with its shorter running time, still leaves room to add songs and maybe even restore some of the scenes from the book that were dropped (such as Will and Jim's shared nightmare of the Dust Witch searching for their houses).
As I said, the sound of Finding Neverland's "Circus of Your Mind" would be perfect for this book, particularly in any songs involving the carnival and Mr. Dark. For the Dust Witch, I'd think something sultry and exotic. Jim and Will's songs should reflect their innocence and disconnectedness from life. The lyrics surrounding the carnival and its members (whether sung by them or other members of the cast) should have a mythic quality. I'd especially love something dark and lyrical sung by Will's father as he researches the history of "the autumn people" as he calls them. There should be room for light and joy in the songs, but the bulk of the musical would have to take on a dark, brooding tone ranging from Jim's determination to be older to Mr. Dark's desire to collect the people of Green Town. Speaking of Mr. Dark, he and his partner Mr. Cooger (largely absent from the film) could have a rousing (if chilling) intro number along the lines of "Marley and Marley" in The Muppet Christmas Carol, although their song would likely have a much darker shade of humor like something from Sweeney Todd.
I'm not very well-versed in the stars of Broadway and other theatrical venues, so this will be focused more on what qualities I'd hope for in the characters than specific actors.
Jim and Will are the main characters, so if the show keeps them at their proper ages (almost 14), there will need to be a group of strong child actors in these roles. I would hope that any show would try to keep these two young because it would kill half of the conflict to make them much older than they are in the book. While both of them are white in the book, I don't think race need play a part in their casting, so long as the light and dark dynamic of their hair/personalities is carried over. (It's a pretty big part of the novel's imagery.)
Will's father should be someone like Joel Grey who can portray a frail man with a strong spirit. He should be bookish rather than physically imposing (although if he were someone with a strong physical appearance who could somehow appear to be frail and old, that would be astoundingly wonderful to see).
Mr. Dark was portrayed by Jonathan Pryce in the film, and I think he'd still be a good fit for the role. It needs a certain gravitas and chilling threat in its portrayal, someone who can play a refined businessman when the need arises but has a deep well of darkness and violence waiting beneath the surface to swallow the unsuspecting. Mr. Cooger would need to be someone who has excellent chemistry with the actor for Mr. Dark and who could hold his own when playing opposite such a large personality.
For the Dust Witch, there will need to be an actress who can express herself as everything from a creepy old witch to a young seductress. She will likely be a woman of color, in keeping with her description in the book.
So what book, movie, or other story would you like most to see turned into a stage musical?