I'd make an action game where you can design your own scripted game scenarios, with infinite possibilities. User-friendly interface with deep customization. You could re-create your favorite action-movie sequences :-D
My game would be something of a cyberpunk robin hood. A futuristic world where your upper class family is destroyed and you form a band of hackers from the slums and run various missions against corporate AIs. It would be an action RPG with an open world, think future GTA with stats and loot.