This musical theatre offering may seem utterly off-topic for a blog about superhero comic books, and it's possible that quite few of the people who like one would like the other, but there is a fundamental connection in subject matter (beings with amazing powers interacting with normal people). The stronger reason for me mentioning it is that like some of the best products of the superhero genre, it is a relatively well-developed, popular, and highly profitable work of art based upon an original work that is much thinner than the derivative. Look at the witches plot of The Wizard of Oz (stripping aside the other more elaborate plots and parables that are not utilized; Dorothy is not seen except in brief silhouette in Wicked) and you see nothing but the barest of precepts for Wicked -- much as if you read the six-page Batman story in Detective #27 and tried to discern the basis for the billion-dollar grossing Dark Knight film that came seventy years later.
The two cases are far from identical: Batman's debut launched a decades-long serial that had thousands of installments in various media before The Dark Knight came along. Clearly, the Nolan Batman films were not drawn simply from Detective #27 -- there is more influence from The Long Halloween, Batman Year One, The Dark Knight Returns and other landmark works in the original genre. The Wizard of Oz is not a serial, so Wicked had only the original to draw upon (in fact, there is reference, much of it for laughs, to such outlying topics as The War on Terror that date the musical according to its 2003 premiere).
But the better Batman works such as The Dark Knight Returns and The Dark Knight also draw upon real-world, grown-up, real life to fill the pages and the frames, and are interesting, certainly to a broad audience, precisely because they speak to those concerns. If they are comic-booky, they are comic-booky not greatly more so than The Godfather or a Martin Scorsese film, providing a fictional and only-so-realistic narrative to make us think about, or at least thrill to, real issues. Wicked does this, too, bringing in philosophic food -- or snack food -- that isn't to be found anywhere in the original, and the relative nature of goodness merges our childhood experience of rooting for Dorothy and against the witch with a credible tale of how the Wicked Witch is actually the best person around, with flaws and crimes that are easy to understand; certainly no worse a person than the good witch, and certainly less flawed except in being green. Wicked also adds a layer of logic to the original, doing something with the visually obvious fact from the original, that the witch is the same color as the city of dreams, and not by coincidence.
When I have contemplated The Dark Knight Returns, and in general all graphic novels that made the superhero genre seem worth serious thought, I have wondered if the exercise, besides being fun, is really warranted. Yes, it is possible to add depth to those stories, but what is it about the original that seems to call for such a treatment? A great story could be written around Dennis the Menace or Humpty Dumpty, but it would only be great by taking a great story and stapling it onto the very thin premise of the original. Giving the Frank Miller touch to something as light as Peanuts is possible, but is (perhaps inevitably) laughable: See this brilliant parody for a demonstration. A social critic might ask why see any benefit in having a great story be grounded in childhood stories. Arguably, The Dark Knight Returns would be just as relevant -- or more so -- if it followed a retired cop who got fed up with the world and returned to his duty though he had never worn a pointy-eared mask in his youth.
That's a philosophical question. In terms of hard cold cash, we have to observe that The Dark Knight was a great material success, far beyond just about any movie that was ever made about any non-comic-book characters. Wicked has been similarly successful in its world, winning Tony Awards and rolling up impressive totals, including some all-time records, in ticket sales. This doesn't make them the best film and musical of all time. There are those who will jeer and say that they aren't even particularly good. Opinions will differ. But there does appear to be a demand to see a story from our childhood come alive in adult terms when we are adults. Little wonder -- that's in general the story of life.