I had a really weird dream that transitioned into a cool idea for a game, but totally out of even my most pipedreamy considerations for actually doing.
I dreamed that I was playing a videogame. I was some schmuck, overweight, bald, in a white t-shirt, coerced into driving a car full of organs for someone who turned out to be a necromancer. Then I was a Guild Wars 2 Elementalist fighting the necromancer with a team. He was almost dead and I somehow got separated from the group, but it was okay because I used earthquake and was about to finish him off with churning earth (a knockdown, then build up for massive damage while he's down). Something happened, I don't remember what. I fell off a cliff and he leashed back to my party with full health. While I was making my way back I got distracted, the leader of my team showed up and got really mad, because I let my team down and also the necromancer was immune to knockdown (nuh uh!) We got into an argument and I woke up.
Most of that description is superfluous but writing down your dreams improves dream recall.
The important part is that beginning.
The video game idea: Grand Theft Auto in a modern-fantasy world. So all the normal GTA stuff; gritty, third-person, free roam, but with magic, vampires, werewolves, monsters, etc thrown in, all secret a la Vampire the Masquerade. Your character starts out oblivious but slowly gets drawn in, first as an associate and eventually as an actual member of this secret society. I was thinking some RPG elements, you pick what kind of magic thing you want to be (mage, necromancer, werewolf, vampire, whatever) with skill and magic trees and such. If you commit crimes you get wanted stars like normal, and the cops come after you. Doing magic shit with public witnesses gets you pentagrams instead, and whoever's in charge of keeping the magic shit secret starts coming after you instead. Maybe you can lower your wanted level by killing witnesses.
This idea is so sweet I can't believe it hasn't been done.