The third, or a merge of all three, seems like a great idea - especially if who's seeking out the weapon is left as a mystery to be revealed through the course of play; along with what exactly the weapon is (good room for twists, I imagine). The idea of being ransomed into helping is also interesting to me, though maybe handled as part of backstory for each character in the individual sense (one doing it out of promised wealth, one bound into it by magic, one for king and country sorta thing). Not sure how that would work though.