Honestly I can't imagine needing more than 6 mk3 Econ stations, but I certainly wouldn't complain if I was allowed to have more.
Some random things I can think of:
player command stations (possibly mil only) generate build points over time, which they can dump into buildings to instantly build them. This would mean that having a military station with a ton of build points available can become an instant defensive wall, if you have enough resources.
Econstations might be able to have a growing resource multiplier, where over time, the economy of the system goes up by a few percet each tick ( say, after an hour or so you are at +30% or something)
a logistics station.. Well, makes stuff even faster later on :/