Fore-Edit: I am intrigued by Spinal's idea and would like to subscribe to his RTD.
Roll to Be a Bandit!
Characters are medieval bandits, with the goal of surviving and evading capture, establishing a hideout and a gang, and, most importantly, earning gold! Players can play individually or ally themselves, hire NPC thugs or work for a NPC boss, with advantages and disadvantages to each.
Every day the players can choose to either go on a raid or do, um, other stuff. Going on a raid entails deciding what sort of target to go after, with what sort of force, and general strategy. The GM then rolls for the target; a higher roll means a better potential payoff in both gold and notoriety, but richer targets are usually better guarded. Then a second roll to see if the raid goes off smoothly; if so, the bandit makes off with the gold and gains notoriety, but if not, a fight will break out. Higher notoriety makes it more likely that targets will surrender without a fight, but also means higher bounties on the players and better protection on some targets.
It's not all raids, though. The players also have to worry about finding a safe place to sleep, getting enough to eat and drink, avoiding law enforcement, purchasing and maintaining equipment, and rival bandit groups. All of these concerns can be addressed with or without money; get a room in an inn, or get some tents, or sleep in a cave. Buy or steal weapons. Fight or pay off city guards. Notoriety also plays into this; if you're unknown, you can just walk into town and buy some ale, but if you're extremely wanted, nobody's going to sell you anything and might even call the guards in on you. Or you could build up a reputation as a bloodthirsty killer or a rich highwayman to improve people's willingness to serve you.
I think that's about it. Might get more complex, but this seems good for now. Anyone interested?