How about this:

A pirate (or rebel) raid on a village has taken everyone they could find to be sold off into slavery. The only ones to escape are a youth who was off somewhere (forests? mountains?) undergoing a pretty strenuous trial as his "Rite of Passage" into manhood, and his grandfather, who was overseeing it. When they get back to the village, all is laid waste. The grandfather spends the next 3-5 years training his grandson in some martial discipline (whatever you want), and then takes the boy to the nearest large town to consult the Guild of Adventurers both to pick up the rumours about (a) who ordered it done and (b) who was the sick b*****d that actually did it.

From there, you can go pretty well wherever you want. You can have fun picking up a party to do it with (consider e.g. the Magnificent 7 and other dramas based on those from Japan), investigating to find the information your party needs, hunting down the perpetrators, and finally bumping them off, either in a full-scale battle, or an infiltration followed by a quiet assassination.

You can even decide whether you want to run sequels, i.e. whether the end-of-game baddie is the actual killer (or set of killers), or the mastermind behind it. Or even whether the mastermind is one of a group, in which case, you have a campaign to plan rather than a battle.

Also, decide in broad lines the composition of the party. You need to have the abilities of the four main classes (rogue, fighter, priest, and sorcerer) covered in one way or another. If you give the players a completely free hand, my experience is that you're likely to end up with a party that makes the game difficult to run because either (a) they're overpowered for the plans you've drawn up, or (b) you've planned something that the party can't cope with because some critical skills are missing.