A recent AskHistorians question about bandits got an answer that is simply overflowing with game-worthy material. The whole thread is fascinating reading, as is an older thread on the same subject.
First: there is a positive correlation between bandits and wolves. There are all kinds of rich ways to integrate this into a fantasy world. Bandits could have some lupine qualities (keen eyesight, sense of smell) or they could be marked visually as wolflike in their appearance. Some bandits might make a totem of wolves or the bandit deity might have some connotations. There is also the possibility of wolves kept as "pets" - or the ultimate twist, that the bandit king is actually himself a werewolf. It's a rich mythic resonance that you can take advantage of when you want to make this gang stand out from the last bunch.
Second: there's a very fine line between noble lords, soldiers, and bandits. In a lot of fantasy there's an assumption that the law is going to protect everyone, but here we find quite the opposite. We saw back in the OD&D setting series that Lords sometimes challenge PCs to combat, so it shouldn't be surprising that sometimes the feudal lord is going to kidnap you for ransom. Tolls and stand-ups for "safe passage" are, of course, a convenient way to relieve PCs of excess loot.
Of course, this doesn't always have to be the Lord himself. Primogeniture means that there are going to be sons who don't necessarily have anything good to do, and they could always get up to trouble. You could even have a robber baron (in the classic sense) hiring a less scrupulous group of PCs as "toll takers" to waylay a caravan - sort of the polar opposite of the stereotypical "You're guarding a caravan" opener for scenarios. It'd also be a hoot for a group of Lawful PCs to capture a group of bandits and deliver them to the local Lord only to find that they are his men working under his protection.
If the PCs are in a stereotypical "borderland" environment, doing some kind of hexcrawl, it's not out of the question to put soldiers on the wandering monster table. These could be essentially bandits, or simply a camp of soldiers who want to turn a buck and charge the PCs for safe passage.
Third: there is the idea of pilgrimage. This is interesting for a whole host of reasons. Pilgrims were the stereotypical travellers of the medieval world. They were typically travelling to some site in Europe associated with a saint or a miracle, or in the extreme case (most common when the Crusades were going well) travelling all the way to the Holy Land.
Shrines and churches with particular holy sites have a ton of potential. A shrine can fall under siege by monsters, or be despoiled by a powerful evil cleric, or just be a location where you have to guard pilgrims. If the PCs happen upon a clerical stronghold, the high priest might send them on a pilgrimage in return for some spell cast or favor done. It's a flavorful way to get people to go from point A to point B. And the motley crew that might be found on such a pilgrimage is the kind of thing Chaucer might tell you about.
Fourth: this is the kind of thing that goes great on rumor tables and guides. "Avoid the bridge over the Sterling River south of the Red Hills, the local lord will rob you blind." Of course, turnabout is fair play, and the same Lord might spread rumors that the northern bridge is inhabited by trolls, driving the PCs south into his territory.
Fifth: the last thing that is fun here is that, this being the Middle Ages, robberies were not always in hard cash. In a lean year bandits or predatory Lords may be more interested in food and wine than in taking hard currency that can't buy chicken scratch. Magic items, of course, are prime targets for a nobleman to demand of a passing hero. And kidnapped characters, of course, can always be press-ganged into doing some adventurous task.
In terms of tone, the idea of bandit Lords and soldiers is an undercurrent beneath a lot of the great medieval literature. It was prettified under the guise of chivalric combat for Arthurian tales but the basic idea is not much different. But if you're ever hexcrawling in the OD&D setting, give robber barons a thought.
