A space game (obviously) where ships are built and broken in convex pieces. With in-game computers and ship design/construction software, of course. Because what would a game engine be without the ability to play a game within the game, with exactly the same visual quality as in the outer one?

Edit: Game-within-game premise based on the idea that it will be easier to write the ship-CAD software in a game engine than from scratch

Edit 2: may as well also be able to play the actual game within itself, since it would be like holodeck simulations