I was thinking less about a prophesy style thing where the result could or could not happen as that would be a logistical nightmare. As for what these arcs would be they could all be based around the "Heroes journey" http://www.mcli.dist.maricopa.edu/smc/journey/ref/summary.html
The amount of evets you describe sounds like a logistic nightmare as well. It'd be rather scripted, in contradiction to DF's free and sandboxy nature. The way you make the story sound also inclines that your story would always be similair, with changing targets and locations. Heck, do you know how quickly civilisations will perish if every adventurer gets a foreboding destruction/takeover of his hometown? Remember, adventurers have the tendency to die every few years.
And one more thing personally I think if you just picked :I want to go get that cool sword: at the beginning it would have no sense of mystery or unknown.
It doesn't need to be a cool sword. You can pick the cat-bone amulet from one of your earier forts and see where it went, what its users did. But yes, you could chose something easy and unoriginal, or nothing at all. But think about your cool sword. If it's that tough, your next adventurer will want it too. And the ones that follow him. Your cool sword will have a whole history of brave heroes and vile bandits to its name in no time.