I have been wondering why many space-colonization games do not include star-system improvements. For example:
1) A network of satellites which communicate with a central computer system through quantum entanglement communication methods which act as a gigantic satellite to look at / spy on other systems. It would have a range of hundreds or thousands of light years and upgrades could allow it to increase the range and resolution. You could use interferometry to cancel out the light of other stars and directly observe planets orbiting those stars. You could use it as a spy satellite system and different sets of satellites could be tasked in groups to observe different star systems, giving you sight of hundreds of stars and their orbiting planets. Any race not much more advanced than our current technology could build such a system. Certainly by the time you are able to travel to another star, you would be able to build something like this and already have scouted which star to go to and figured out whether the planets there could support your life. Sending scout ships is just idiotic.
2) A large network of defense drones / satellites around planets or entire star systems to defend against attacks.
3) Dozens of star bases around the solar system.
4) Artificial moons or planets.
5) A Dyson Sphere. This one is really interesting as your population could migrate to the sphere as it was built and the planets could be cannibalized for resources. The resulting structure could hold probably dozens of planets worth of people and act like a giant starbase at the same time. It could produce food, research, and production, and house enormous amounts of defensive structures and shielding. It would take an entire armada to capture or destroy one. You might assume it would take hundreds of years to build one, but if you have the technology to build one at all, you also have AI controlled robotic construction drones which could cannibalize planets and automatically reproduce themselves and build the structure for you. As the structure neared completion, they would automatically cannibalize themselves and add to the raw materials for the structure. Thus, the rate of building the structure would stay at 0 for a while until enough drones were produced, then a bunch would go start building the structure while the rest continued reproducing. The rate of building would increase exponentially until all planets were cannibalized and then it would hold a very high rate of construction for a year or two and then as it neared completion, it would drop sharply as the drones cannibalized themselves.
There are tons of other options you could choose from at various different levels of technology. All planets in the solar system could contribute all or a portion of their production to build items in the solar system build queue.