It is traditional for two warlike countries to each give their first-born royal prince as a hostage to each other to prevent war. One of the princes has been murdered, and the PCs have a few days to rescue the prince in the other country to save him from being executed and a bloodthirsty war from breaking out.
A certain country allows most civilians to carry weapons, but only if they are potentially deadly. Pepper sprays, stun guns/tasers, batons, and dart guns are banned. And if you kill someone and are not acquitted at trial, the punishment is to be hanged the day after you were found guilty. The message here is that if you want to go armed, you had better be prepared to risk killing someone and being executed unless it was clearly self defense.
When it comes to combat spells, again, only the highly dangerous can legally be cast with the same possible consequences if cast wrongly.
Many weapons buried with the dead are buried for a very good reason as they have had a potent curse placed on them making them dangerous or near-useless to the living.
A certain tribe in a valley believes that the soul remains trapped in the body after death and is aware of it. So they treat their dead as if they are alive, talking to them and keeping them at home. They are used to the stink and for some reason are also immune to the diseases that this practise causes but outsiders are not so lucky.
An army and navy who work in the same way as others except their rank titles are upside-down. So people start as full generals and those with the rank of private command armies.
In a deeply unsettled world, much of it anarchistic or ruled by murderous gangs, a group have adopted the armour, weapons and discipline of an army of the distant past.
What if casting magic changed (for a few hours or days depending on the strength of the spell) the colour of the skin of the spellcaster? It could lead sadly to a very racist world to rp in.
Have a large room that elevates either up or down when triggering a trap. The floor can bring the players to new levels, where a mighty beast roams, or the floor flattens the players into the ceiling or drop into a pit of lava.