lol something to think about. The only thing is that if I replaced a planet with that then it would spawn multiple times throughout a solar system since you can't make a new planet in game. This is something I would never do (make a machine type planet) since it's unrealistic and not natural. What I am thinking of doing however is making a planet ring that is buildable in game that is round and goes around the planet it's built on, giving it extra population. I'm not sure if this is possible but I will give it a shot.
Just a thought, but as easily, or difficult really as it is to change the mesh for a planet to a ship, for your mechanical world, we could go the other way, make a ship, not actually a ship, but lets say starbase, since they are the closest to planet in their ability to spread culture, spawn ships and fighters and mines and are in fact mobile, or can be made mobile and resize it, to what a planet is. with maps you could place one for "your world" and leave all the traditional planets for the opponents and across the system. The difficulty i see, is making it so the ai recognizes it as a colonizable body, and therefor legitimate as a planet for your race. It would be nice to be able to "create" new entities that are "hybrids" using part of one item and another without "breaking" any hardcoding. But that is probably too far gone now. But might be an interesting direction, if Blair and Craig and Company moved Sins into a more modular direction with each new expansion, replacing hardcoding with "plug and play" modules.
It would mean being able to create ship planets perhaps, or colonizable ships, or cross-making new abilities or introducing other "cash" elements say beyond credits, metal and crystal, we could add, fuel, or ammunition, or dutuerium ore, or any dozens of other "craft" items that could be used as a basis of "building" something new, as in iron and carbon to remove impurities and produce steel. In the same way, creating dozens of craft items to mix and match to produce "different" items in game could be used to produce lets say, self-sustaining shields with no energy expenditure, but of course such an item does need a downside to balance it, say a constant projector ship, that is "rented" not bought, so the credits are continually in some pre-determined amount are deducted from the total. When the credits run out, the ship vaporizes and our wonderful self-sustaining shields with no anti-matter drain are suddenly gone.
My point is yes, you are right about certain limits being in place now, but it could change. HW was "The" Standard by which most modern space RTS's are judged. But Sins, with some creative and interesting directions, could be the "New" standard, and it might of course be possible to go beyond just animated turrets and an interesting storyline, but could extend into making "craft items" or a more "modular" design moving forward, or a player-centric campaign editor, or "plug and play" parts of abilities that can be joined and re-joined in many different ways producing different and new abilities.
Just some thoughts, i hope i dont sound critical, that is not my intention, i just wanted to say we or the devs, or both, the devs and the player/modding community might be able to do some wonderful wonderful things moving forward. Not now maybe, but a year from now? Two years from now? Where could we all be if we had these kind of goals or ideas and directions to shoot for?
Ten years from now, new space RTS games might be compared to Sins, not HW, if Sins goes beyond and in new directions with this game. 
Sincerely,
-Teal