I don’t have a ton of nightmares, but I tend to enjoy and remember them when they happen. I had a nasty one the other night that is surely rife with some gaming inspiration.
When I was growing up, I had some cousins that lived in in a neighborhood of row houses, very urban with minimal front yards and long narrow back yards. The dream was in that type of neighborhood, and a group of loud nihilists moved in next door. They had outlandish hair styles and seemed to be into extreme piercings.
The horror part of the dream started when all these folks started maiming each other, cutting off body parts as tribute to the 'god of pain', the maimed one. Hands and fingers and ears were chopped off, males castrated themselves, tourniquets and cauterizations were done so they could stay alive just long enough. "Our parts don't matter, since none of us are coming back from this," they were saying. Then they went off through the neighborhood with knives, hatchets, and samurai swords, and started cutting, chopping, and slicing everyone they met, dragging people from their homes.
At that point, the dream degraded into a bit of chase, a really common trope in nightmares. We ended up at the beach (a fast change of scene that only happens in a dream) and were hiding in a boat house that stretched into the surf. Except "Jaws" was in the boat house too, sneaking under the seaward end, and we wouldn't dare go into the water or take a boat with the head of a gigantic white shark munching everything in sight, beached as it was in the surf and stuck in the boathouse. That part was too nonsensical, and then I woke.
The bit about the devotees of the maimed god was pretty disturbing. Dreams usually aren't that visceral, with people cutting off their own noses and ears and other parts and then going on a murderous rampage. A rather grueseome gaming scenario could be constructed around "the new cult that moves into town"; it seems innocent enough at first, but as members get indoctrinated into the belief system and suppress their will to the cult, it builds into a murderous rampage of self-inflicted trauma and carnage. Sadly, murder-suicide has become so commonplace in the world that such a scenario might intrude too much reality into one's fantasy game.