I like this. I actually once structured an entire campaign around a similar concept, with the PC (it was a solo campaign) being fed a fairly standard fantasy intro. There was an evil empire, they were toppled in a civil war and an uneasy peace has come across the land as the evil empire threatens to return...
By this point, the PC is fairly confident (from a metagame and storytelling standpoint) that she's going to spend the game fighting the scions of the evil empire returned. Pretty standard storyline. Imagine her surprise when, while investigating rumors into her own past, she discovers that she IS the last remaining descendant of the evil empire. Her duty isn't to vanquish the old world order, but to reforge it. What she'll do with it when she has it, now, that's the real question.