|OMG - Did you hear what the magic-user said about the illusionist?|
That being said, there is no reason why settlements cannot present new and interesting challenges to the players. Challenges is the key word here , not dangers. Challenging NPCs can make a visit more interesting without killing or harming the characters physically.
Here are a few ideas for obstreperous NPC's to bedevil the player characters ...
1. The Spy – Always nosing into the adventurer’s business, and selling their secrets to interested parties (i.e. the man in the shadows). Spies can help move an overarching plot that develops slowly, and eventually become a source of adventures.
2. The Gossip – Spreads rumors and gossip around town concerning the adventurers, and not always that accurately. The gossip can tip off rivals* and piss off neutrals and allies, making life more difficult and expensive for the adventurers.
* Rival adventurers, of course - I strongly suggest rolling up one or two rival adventuring parties. They use the same town as their base of operations, and thus bump into the adventurers in the local tavern and compete for hirelings. More importantly, when adventurers are trying to clear out a dungeon, the rivals might get to key rooms first, or even run into the adventurers as a wandering monster.
3. The Buffoon – An idiot who wants to help the party, but his help always turns out to hinder (e.g. Gilligan or Joxer the Mighty). The idiot really does mean well, so non-evil characters may have a hard time getting rid of him.
4. The Braggart – A loud-mouth braggart who challenges the PCs at every turn, but couldn’t possibly handle him- or herself in a duel. Unfortunately, the braggart can make things tough on the PCs if they fight back - perhaps their father is wealthy or powerful.
5. The Nemesis – Counts one or all of the PCs as her enemies, to be crushed, destroyed, demoralized or generally messed with. The reason lies in the past, and may be an event so minor the PCs don’t remember doing it. Naturally, the nemesis does not attack openly, and may even appear as a friend and ally. The nemesis attacks through others, and has friends in high places.
6. The Fanatic – The PCs biggest fan, a person with no life of his or her own who has latched onto the PCs, living vicariously through their adventures. The fanatic brags about them, which can create problems with rivals, and begs for chances to adventure with them. If the fanatic's illusions are shattered, they will turn quickly against their former idols.
7. The Mixer – Likes to start trouble, especially between the adventurers and their allies or retainers, or within the ranks of the party itself. The mixer is cunning and seemingly harmless and innocent, and almost always poses as a concerned advisor and friend.
8. The Schmuck - The schmuck is genuinely likeable and good ... and completely hopeless. They are always in need of money or help because they bit off more than they can chew (gambling debts, a fight with a bully, trouble with the tax man or loyal aristocracy, etc.). No matter how much the adventurers help, the schmuck will always screw it up or require more help.
Think about introducing one or two of these annoyances in your next city adventure, and spend enough time on them to make them worthy of being reoccurring NPCs in your game.