A multi-player video-game where every player owns a few planets which orbit around a supermassive black hole (But they are far enough to avoid spaghettification). The planets are Minecraft-like worlds (Made of cubes), but instead of being plain, they would be in a giant cube (Spheres are difficult to make). (The planets can be spheres, we can make the voxels come together to make spheres. Or maybe have the voxels only when the player is close enough or something) The planets have different resources and the players could create tools, furniture, boats, planes and even spaceships.
The aim would be to create a few black holes systems, which orbit around a common center of mass. Some black holes are solely with the purpose of constructing and marketing, so the players can build their dream worlds.
Other systems are governed by different factions (Do you remember Alliance and Horde? World of Warcraft?), and its players can build their worlds, protect them and go fighting to the enemy worlds, in order to gain reputation and materials.
The spaceships would behave quite like real ones: For example, they do not go in a straight line, they have orbits. But in order to do things simpler, they would have infinite fuel, they do not have atmospheric drag and the orbits would not be with N-Body physics, just around a single planet or black hole, like Kerbal Space Program (We have to manage the Spheres of Influence). Maybe we can add galaxy exploration later on, when the game is mature enough.