Build a space base, or station, or dungeon or architecture in general. I could launch your app, hop in, setup the space inside as i see fit, make changes as i go along and when i quit, the setup is saved, and then i could add custom things manually ( decals, extra detail meshes, special lights, etc).
Perfect would be if you could give voice commands. “Make Room A”, “Make Corridor B”, “Make Entrance A”, “Make Window F”.