Add other space objects, combat, missions, etc
You could add other space objects such as Moons for other planets, and asteroid belt Maybe. You could also add a combat system see you can fight other players, AI or even aliens! You could then add in campaign mode a space race setting where you could race against other players or AI to discover things like the Moon, Mars, excetera the fastest. In my final idea for now would be to make it so that if you beat the campaign mode you could restart with a slightly different solar system, adding new planets, starting on different planets, excetera.