I like the idea of space colonies, maybe having a maximum of modules allowed to be built on it. Each module able to hold a unit of population, and acts as 1 sector. Different modules act as one game,but in theory could be separate bases acting as a single colony. Maybe they act separately in defense too. (as in they add multiple bases to the fleet in that sector) The drawbacks that could be placed on them as opposed to planets are that natural resources, while native to planets, must be shipped in on trade ships and supply vessels to a space colony, making maintainance costs much higher. Food should also be more difficult to produce on a space colony. (Most are too far from a star to have adequate natural lighting for vegetation, and artificial lighting would be a drain on resources) However, ship construction in a 0G environment would be ideal, so that would have a considerable bonus. Plus 0G research would be easier to find in space than in orbit, after all, orbits around planets eventually decay if not maintained, and a space colony would have a resident population that doesn't need to be transported to orbit for work.
Basic space colonies could be a low level tech, which includes habitation, manufacturing, and research modules at maybe 75% efficiency and quadruple maintanance. Later improvements could eventually improve them to maybe around 150% efficiency with a slightly decreased maintainance, maybe to about double maintainance. They should never approach the maintainance levels of a normal planet, the rules of economics in resource scarcity dictates that. A good ways up, you could add artificial gravity to habitation modules, and even agricultural modules at low efficiency. Also, a space colony could start with room for 2 modules, a habitation module, and 1 of your choice, while later on higher techs allow for maybe 2 or 3 more modules added. At that point you could consider them expensive planets wherever you need them.
Practical uses would be research outposts that are difficult to be discovered. (Since it's artificially created, less real estate is needed, and will be nowhere comparable to the size of a planet, maybe the largest would be a small moon) Fronts lines supply/refit base. Frontier war machines. Later on they could bring revenue once they become less expensive. Finally, populations shouldn't grow too quickly on starbases as people aboard will feel less "free." Therefore they could be used as a place to move extra population on overpopulated planets.
As for Dyson Spheres or Ringworlds, those would be the analog of creating a new planet. A Dyson Sphere would not "need" to be build around a star however, just around a light and heat source. It could be artificially generated. Also the edges where the "gravity" is weakest (remember, unless you have a way of generating gravity, artificial gravity must be generated by spinning a ringworld or Dyson Sphere around the center) you could have production and research centers, while the center would be good for agriculture. A ringworld on the otherhand is taylor made for habitation and agriculture, but not very well for production or research, as you don't have easy access to 0G zones like in a Dyson Sphere. I think that even a ringworld could be build on a smaller scale with an artificial light and heat source. Of course not using an actual star would add a new expense to the maintainance costs for the constructs.
