Hmmm. You've given me a lot to think about... I like the shadow of collosus + restoring water to the land that was horded by a tyrant idea. It's a simple reason to build many levels, but around an overworld map. Might be a bit ambitious though - since an open world demands a lot more tech, and I am one person.
Also just FYI, the killing of the king is supposed to be like the flag at the end of old mario levels. Just a satisfying finish-line ritual - but everyone was expecting a boss fight it seems :P