I have idea with was brought by constructive criticism in thread about engineering points.
The idea is about allow to build production projects called specialized factories. Specialized factories would be building with upkeep cost in:
1. Science points (we must train labors to work in specialized factory)
2. Gold (we must paid to labor for work in specialized factory)
3. Shields (labors hired in specialized factory doesn't produce other thinks) - this last point is to discus
The upkeep cost of specialized factory depends on level of it - the count of unit it produce in one step.
Number of specialized factories build in city will be limited by population.
How to build specialized factory? We can select to build specialized factory from production list. In next step we select unit specialized factory might produce. In next step we could select how many units specialized factory would produce in single step. Shields needed to produce specialized factory depends on number of units we prompts. It will be linear to number of units.
After build specialized factory we can select new build project from production list. The project is assigned to specialized factory. The specialized factory(after selecting project related to it) will produce as many units as we selected in adds specialized factory to production list process. We can build all units of count we provide in this process or none units, so stopping producing units isn't good idea. Cost of producing each unit in project is reduced linear to number of unit produced by specialized factory, but it cannot be greater to settings written in units.ruleset for unit type of produced unit. This value could be called "weapon cost". It would be greater for armored units and lesser for units like warrior.
Why this feature? Because it's realistic that factories produces weapons and mass production will decrease production cost of part. It will force player to produce specific units in specific city(because allowed maximum number of specialized factories depends on population and player would produce many units at once, so they cannot locate all specialized factories in one city).
To discus: Should specialized factories give free upkeep for units of type it was related? Of course - only, when not producing.