Come up with some interesting way for members of certain classes to settle differences. Maybe merchants who disagree about prices or shipping schedules or whatever have to duel with padded quarterstaves on a narrow plank over a river. Starship crew who get into arguments settle their feud in a holographic showdown with real electric shocks. Superheroes who have a grudge face off in a no-holds-barred battle in the ruins of a ghost town, so they can destroy the buildings and hit each other with cars with no consequences. Wizards hold ritualised magical duels of alchemy in which they prepare potions that the other must drink.

These all add flavour to your game world. And give you lots of good justifications for getting characters into grudge matches with NPCs!

Transcript

Luke: We need that ship!
Yoda: Need it you do not.
Luke: No, really... I do!
Yoda: An argument we have! Ah ha ha ha! Settle this like Jedi, we must.
Luke: What? Swords??!
Yoda: Be silly, do not! A Force Arm-Wrestle we shall have!
Yoda: Hubert and Nancy we must levitate! Only three days hike away they are.
Luke: What?
Yoda: The two largest boulders on the planet they are. Dibs lifting Hubert first! He's been on a diet!