The idea of "destiny" is tossed around a lot in books and movies, especially in epic, heroic adventures. We have the "destined and doomed" lovers Romeo and Juliet; the "destined hero" Luke Skywalker; the "chosen one" Neo in The Matrix; the "Boy who Lived" Harry Potter.
The designated hero in an epic adventure often has a really hard time avoiding destiny, and Elphaba is no exception. We know her ultimate fate before we know anything else about her. In fact, the way we have to read Wicked plays an important role in the theme of fate and free will. We spend the entire book aware that Elphaba is doomed and her free will isn't ultimately "free" at all. This becomes one of the major questions of the book, asked by Elphaba herself: Has her entire life been predetermined? Was Elphaba's death fated to happen, or did her own choices cause it? And can we read Wicked as a tale of destiny and doom, or as a sort of the ultimate explanation for her death?