Gregory Maguire is a huge (and self-professed) fan of T.H. White, who wrote The Once and Future King. Maguire had this to say about that book:
I observed in it several admirable attributes that I try to make hallmarks of my own work. First, the book is derived from a popular set of myths and commonly held stories that form part of our Western foundation myth (the King Arthur stories). Second, the book is by turns profound, endearing, and comical. (Source)
Maguire explained in an interview that he based Elphaba on some of his own favorite real-life heroines:
[Elphaba] is my hero. I wrote her because I wanted to put in the parts of my heroines – Emily Dickinson, Lena Horne, Virginia Woolf, Laura Nyro – all in one person, and have someone to admire.
Emily Dickinson was an American poet and noted recluse. Lena Horne is a singer, actress, and civil rights activist who appeared in the 1978 movie The Wiz (yet another adaptation of the Oz myth). And Virginia Woolf was a modernist author. (Source)
In their "100 Great Heroes and Villains of Film" list, the American Film Institute voted the Wicked Witch of the West the number four villain. Good job, Elphaba. Who was number one? Hannibal Lecter from The Silence of the Lambs. (Source)
L. Frank Baum, author of The Wonderful Wizard of Oz, actually wrote fourteen Oz books between 1900 and 1920. Maguire draws from the Oz mythology introduced in these later novels in Wicked. (Source)
Maguire took Elphaba's name from the phonetic pronunciation of L. Frank Baum's initials: L.F.B. (Source)