A lot of my games sort of take place in the same universe, even when they're different systems or settings.

Like an old DND campaign had the players visit a wizard university, where they met many NPCs. One of them was Reg. He's kind of a chill party dude. Loves playing wizard pong (it's like ping pong, but with mage hands)

My current game is a 2050s corporate dystopia using Fate. Heavy inspiration from World of Darkness and Shadowrun.

And Reg is here. He fully believes he used to go to wizard school, but something happened and now he's here. He's pretty chill about it, though. Last game, a werewolf was going berserk and Reg was like "Dude. Fucking metal." The werewolf gave him a knock-on-your-ass high five and Reg lived.