I predict someone would want to kill me for that suggestion, but I would make it anyway.
It is not a secret, that extensive station spam slightly change ruins beyond recognition a game balance because of market concentration in a single system. So suggestion is to have "soft" cap for same type of stations in a same system, but a "hard" cap for overall station quantity per system.
For the "soft" cap, I mean if player wish to build two (or three, or four...) mining stations, he could, but each next would decrease ore/rare ore potential for all mining stations in a system. Same to siphons, player could build up to three of them around each gas giant, but second and third will decrease amount of volatiles on all of them. Before you say that this is "opposite to balancing" thing, read "hard" cap section.
For the "hard" cap, I mean total amount of stations player able to support in a system. I suggest it be number of colonized planets in system (not stations) +2, but no more, than 8 (or so. It better be adjustable). Inside that "hard" cup player able to build any type of stations in a system (in a proper conditions, off course), he need (with respective penalties to production if "soft" cap exceeded). So player able to build more stations of a same type if he can't/don't want construct other type of stations. Also, that would force player to colonize planets if he want to support more than two stations and make prioritizing decisions what he need more: mining stations, siphons or astropolises. Also applied to recolonizing abandoned stations. If mod ever would have other station types this will apply to them too.
With a single exception to that rule: gate station. So if player already have maximum of stations in a system, and only then achieve gates tech, it is still possible to construct one.
Now hit me with tomatoes! ©