Universe Sandbox is the game I have been waiting for. But I feel like there could be so much more to this game. I absolutely love the idea of creating my own solar system and being able to manipulate anything I want. I also love the idea of being able to explore my creations and make them truly unique. Here are my ideas:
I would like to be able to customize a planet or moon to my liking. Maybe in the hypothetical planet editor, you can decide what the planet is made of (which will determine the properties of a planet), whether it is geologically active, if it has a thick atmosphere, etc. In the editor for a rocky planet, the planet's surface is covered with a grid of hexagons. Each hexagon can be filled with 1 of, say, 4 basic geologic structures: land, water, mountains, and volcanoes, which will allow for easy creation of seas, islands, continents, and mountain ranges. In the editor for a gaseous planet, you would be presented with a variety of possible substances to be used in the atmosphere. A similar hexagon grid system would also be present in the gaseous planet editor, where each hexagon can be filled with weather phenomenon like wind, storms, clouds, and hurricanes.
The ability to seed a planet with life would be fun and interesting. Fauna and flora on planets would be dynamic and ever changing. Occasionally, one might see artificial satellites orbiting a planet and spacecraft wandering the solar system when a planet's fauna becomes advance enough. Maybe if some sort of a planet explorer is implemented, when exploring a planet with life, you might encounter various creatures or possibly even primitive villages or advance cities.
Introducing the ability to create gas clouds and nebula would allow for more artistry in solar systems and would provide an interesting way to create planets. Instead of spawning planets and stars, one could create a huge gas cloud and make it spin to create newborn stars and planets. It would also be interesting to use to visualize the development of planets as you would be able to see the path a young planet cleared in the gas cloud while orbiting.