Consider the sorts of things which are lacking in a frontier settlement.

• Established law & order: I say established, because law & order will always exist in human(oid) settlements. The key question here is "who controls the law?" There'll be a sheriff, but it's up to you to decide if he's in the pocket of the local baron, a righteous / unflinching do-gooder, or a worn out drunk who barely maintains a semblance of peacekeeping.

• Health and medicine: Outbreaks of plague and disease could be devastating. Think about the town's water supply and its sewage system. Any overlap in the two, either via shoddy planning or simple geography, will mean disease. Furthermore, while the town will likely have food stores, medicine is less likely to be available, and may need to be brought back from distant, more established settlements.

• Fair trade: This is a big one. It's almost certain that elements of organised crime are present. Maybe not fully established, but there will certainly be things like price fixing (especially for sought-after items), scams, and forcible removal of inconvenient competition.

• Religion: Religion likely won't be lacking, but its institutions will be nascent. There may be a basic temple or shrine, but it won't be ruled over by a powerful priest, but rather a travelling minister or preacher, a missionary (or more, from various religions / gods) who's come to minister to the settlers and perhaps convert locals.

• A military: This does depend on what sort of frontier town it is. It could well be a military outpost, but it's more likely to be one founded by intrepid settlers and entrepreneurs. The lack of an established army leaves the settlement vulnerable to large scale hostilities, especially by organised opposition. A few raiding bandits or overly aggressive orc clans is probably okay, but an organised force is a definite threat.

Then consider the sorts of things which are more likely to be present.

• Entrepreneurs: Settlements tend to spring up around abundant natural resources. It may be gold, it may be timber, or it may simple be fertile farmland. Consider what sorts of resources are present, and the sorts of people who have come first (risking the most) to try to exploit them. Who owns which claims, or plots? Who's struck big and who's bottomed out?

• Gambling & prostitution: Depending on the type of campaign you're running these may well be present everywhere, but if not, then they're far more likely to exist on the fringes of civilisation, especially in towns which tend to be heavily populated by men.

• Criminals, outlaws, renegades: It doesn't have to be people who are actively practising banditry or other crimes. It can simply be people who ran afoul of the law back in civilisation and fled to a frontier town, keeping ahead of organised law and bounty hunters.

• Bounty hunters: Come to pursue fugitives who are fleeing the law, and bring them back to justice. Or maybe just to see what sort of mercenary work is available, as guards, or even something more sinister ...

• 'Natives': Again, this depends a lot on the type of settlement it is, but depending on relationships with the local native population, you may well have established trade, communication, or even cohabitation with them. Perhaps the local Hill Dwarf clans trade ore for grain. Maybe the native tribespeople have begun to assimilate into the culture, lured by booze and women and other vices.

There's plenty of really cool ideas for adventures, but the best part about a frontier town in my opinion is just letting players loose in it. Draw up some key NPCs (de-facto mayors / government, sheriff(s), merchants, priests, bounty hunters, outlaws, innkeepers, etc) then let players explore. Throw in an event if you want (plague, bandit raids, etc) but let them sandbox as much as possible. Have some feuds in place between key NPCs and let them pick sides, etc.