What if the Wicked Witch of the West was just misunderstood? So posits this lavishly designed reimagining of the cackling baddie from ”The Wizard of Oz.” From the novel by Gregory Maguire, the musical follows the green-hued Elphaba (a pull-out-the-stops Idina Menzel) from her social-outcast school days to her fateful encounter with that girl from Kansas. A few other familiar faces turn up along the way, including the bitchy blond Glinda (a glittering Kristin Chenoweth), Elphaba’s nemesis — turned — bestest friend. Written by Winnie Holzman (”My So-Called Life”), ”Wicked”’s at its cleverest with the girl stuff; the streamlining of Maguire’s story — reducing a totalitarian allegory to an animal-rights flap — weakens the narrative logic. Catchy Stephen Schwartz songs (Chenoweth steals the show with ”Popular”; Menzel takes it back with ”I’m Not That Girl”) compensate for a dreary opening number, but it’s the ”Wicked” witches who really make magic.