The Wizard is in a position of power where he has spent a lifetime misleading the public and frightening his citizens into submission. A little girl from a far-off land approaches him, asking for assistance, and his response is to send her on a mission to kill his most dangerous adversary. In return he makes promises that he’s incapable of keeping, giving snake oil presents to Dorothy’s helpers and then escaping before fulfilling his promise to Dorothy.
1. Time is a slate–anything can be can be changed! Be very careful, you might prevent your own birth. (ala Back to the Future). Paradoxes are a major problem–if you change antyhing you could prevent yourself from going back which would keep you from going back to prevent yourself from going back–and so on.