It is said that though the crown has always been passed through inheritance, no man may rule without the approval of the Archmage and the Guildmaster. (Interpretation I leave to interested parties.

One of the standard peices of equipment for an adventurer is a sword. But swords were actually very complicated to manufacture. Therefore, an adventurer who owned a sword might have had it passed down through the generations or might have been rewarded by some influential figure. Or simply looted it from a tomb.

The ruling family depends on some magical artifact or place to secure their right to rule. A monarch isn't considered legitimate until he or she has been "accepted" by the artifact or place, whatever that might entail. It is also possible that some one else, not of the ruling family, could be accepted as the legitimate heir... A fact which the monarchs would like to keep secret.

Perhaps those that practice elemental magic begin to take on characteristics associated with their chosen element. For instance, an earth elementalist might be prone to agoraphobia, while air and possibly fire elementalists might have problems with claustrophobia. Water elementalists might always seek the path of least resistance. A fire elementalist might have a cat's opinion of water. This could also apply to physical differences. Fire elementalists might have a freakishly high metabolism and a permanently high body temperature. Water elementalists would probably never get dehydrated, but might slow down a lot when it's cold. Etc, etc.

In cases where a person has accused another of a crime, but there are no other supporters of the accusation or defenders of the accused, and there is not enough evidence to support one party or the other, the two people involved, or their champions in some cases, must duel in the presence of the local ruler or leader. The loser is assumed to be lying and punished accordingly.

There is an important pair of fraternal twins. Possibly minor dieties, sorcerers, or maybe just an important pair of nobles. One is evil and one is good. The thing is, the one who looks evil at first glance is the good one, and vice versa.

There is a group that believes no good can come of trying to change to future to render a prophecy or prediction incorrect. They believe that meddling will either cause the prediction to be true in the first place, or make something worse or more extreme happen.

There is a person who has a well-known reputation, probably involving something illegal. But this person is either very clever and crafty, or really like his or her privacy, for no one knows who this person actually is. As an added twist, perhaps this person is of an age that would noramlly be wildly inappropriate (say, a child is a much-feared bandit) or maybe not the expected gender or even race.

If a creature is transformed into another creature (say, a human into a dog), then that creature will become more and more accustomed to the new form until a point is reached where reversing the spell is impossible. A second transformation is possible, but the creature will have to go through the whole process again.

It is not considered a crime if a ship's crew mutinies against a captain that is obviously unfit for the post (dangerously incompetent, insane, or overly cruel). Assuming, of course, that they can prove it.

Rivers and other natural boundaries are used as political boundaries as well.

Atheists and agnostics get a jolt when one or more deities make an appearance, and take a direct interest in things. It is hard to deny the existence of gods when one has met them face to face.

Bards and minstrels, if business is slow, will moonlight as freelance diplomats, couriers, or spies.

The army relies on a loosely organized, semi-freelance corps of scouts for information-gathering. The officers in this scout corps are usually retired spies.

An ancient, psuedo-immortal race has been manipulating something important (politics, religion, economics, etc.) for centuries. Hardly anyone is aware of them, let alone their goals.

One of the most wanted assassins in the realm looks exactly like an important person (local nobility, PC, etc.) This may or may not be common knowledge.

There are two major (read feuding) provinces in the kingdom. The Lord of one is secretly a mad, power-hungry despot, constantly plotting against the king. The Lady of the other is secretly an ancient silver dragon.

In addition to the Thieves' Guilds, there is a hierarchy that all thieves are part of. Rank is based on skill, and can be raised or lowered at any time. The Queen of Thieves, the greatest living thief in the world, is the ruler. Guildmasters answer only to her, and she answers to no one at all.