You play as the forces of Evil Weirdness. Nyarlathotep, the Demiurge, a Ruinous Power from Warhammer - whichever abomination tickles your fancy, as long as it works subtly or from a distance. Your goal is to spread Magical Chaos and Eldritch Awfulness across the world. You do so by creating the dungeons/adventure locations, monsters, magic items, supernatural phenomena, etc. that the players usually fight or otherwise try to deal with in games like D&D, Call of Cthulhu, and other fantasy/horror games in which evil is usually fought by small groups of heroes, adventures, or investigators.
You start out making isolated pockets of Evil Weirdness. These pockets often seem unconnected, partly because you want them to appear unrelated to each other in the eyes of all the potential investigators out there to prevent them from realizing a greater conspiracy is at work, and partly because your agents on Earth (or whatever world you're corrupting) are varied and don't always get along. Over time, you can start making the isolated bits of weirdness start working together to ruin the world. You can amass cults, build armies, corrupt the laws of nature or reality, influence world leaders with dark magic, weaken the foundations of society, and just generally start partying like it's 999.
You are opposed by pesky do-gooder types among the human/humanoid population, monsters and magical phenomena that don't want to cooperate with you, other gods of Evil Weirdness who want to end the world their way instead of your way, dwindling resources (what, you thought Nyarlathotep didn't have to pay taxes?), and any progress or improvement that society manages to undergo over time. And maybe aliens, because aliens are cool.
There are some similar games that already exist, like Dungeon Keeper and Cthulhu Wars, but I have little experience with these games. I'd like to see a game in which the goal is to spread chaos in a way that is both worldwide in scale and opposed by small, independent groups of resourceful people. A Sauron Simulator, if you will.
I don't think I'll be able to pull off designing such a thing, or at least not any time soon. If someone steals this idea to make a good game, I'll be more than happy.