A space builder (4X) where you play as the Emperor, the focus is on macro not micro (you don't build planetary infrastructure manually (it's not allowed), it's constructed automaticly by the AI).

You can affect what is built (like if you want more food and require more farms) on a planet by 3 ways:

1) what kind of planet you colonize (a desert planet will never build farms :))

2) you can set specialization of a planet (food, mining, industry, refinery, administration), such planet will tend to build more of the requested buildings

3) you can enact empire wide laws, decrees, edicts that would affect the production/infrastructure

The question is, how the 3) can be done? Note: it's OK to make it more complex if needed (the game is from the point of view of the Emperor that rules hundreds stars, so a more complex laws system is compatible here).

To make this topic rolling an example solution: lame, overused and unthematic sliders "agriculture slider", "mining slider", "manufacturing slider", "refining slider" :) Then the AI adjusts construction of new buildings by the position of these sliders.

1) what kind of planet you colonize (a desert planet will never build farms )

But...I like my moisture farms....

I think that rather than directly setting the amount of agriculture, etc. you can issue an edict that grants positives or negatives to planets for following or not and and then have the planet decide if this is enough of a benefit for it to follow. For example the bonus might be donation for special civil buildings for food production, if the planet doesn't care about civil buildings but can easily produce more food then they would follow any way but if it is going to be difficult (eg. no space and have to remove other buildings) then they won't follow, on the other hand a planet that values civil buildings largely may take be willing to try harder to earn the bonus.

You could also have edicts that allow for greater amounts of pollution on planet types (or less) that would influence planets to develop more or less industry for example (or produce agriculture if this counts towards reducing pollution).

(a desert planet will never build farms )

And why not?

Who says that a farm must only produce food? Ever read dune? :)

On a more serious note, how about you just set your policy as a set of laws with tickboxes, in the same way simcity 3000/4000 do with the local bylaws like 'nuclear free zone'. Various combinations of ticked options would provide for a massive number of possible outputs (different results and playing styles) with very little coding up front needed by you and very little impression of micro management.

Guilds could be a fun way to theme up a slider system. The Farmers Guild tries to obtain subsidies, research grants, etc. for agriculture. They try to get planets designated as "priority farming worlds". The miners guild does likewise for mineral extraction. The Entertainers Guild for happiness granting structures, etc. As Galatic Emperor you give or withdraw your support to a guild to change how influential it is. You could order your propaganda arm to teach the worlds how important farming is. Or you could have the head of the miners guild framed if he's grown too powerful. Events would shift guild power outside your control, to try to give a feeling of guiding the fate of the galaxy, but contending with conflicting agendas.

If there happens to be a balance issue with focusing too narrowly on one aspect of development (e.g., turtilng and going all tech focused is over powered), you could use corruption as a countermeasure: As guilds become more and more powerful, corruption and bureaucracy consume more of your resources.

I really like the elegance of Brain's suggestion. Passing laws should be very blunt instruments with some kind of thematic hook. IE: A Greener Tommorrow which is about focusing the entire empire on environmental conservation (with a huge variety of effects and impacts). Then you begin mixing these things together "Nuclear Power In Every Home" which has its own impacts and after a couple law passings you have a very weird place full of mildly conflciting policies that is radically different from where you started.

I realized there are actualy two things here: a product and a building. Like if there is a food shortage it does not necessarily mean the player has to/wants to increase the rate of building new farms... Instead/in addition there could be a law like "subsidy food production - it makes farms +10% more efficient".

Actually, the key is to give the player the tools to deal with a problem (food shortage, minerals shortage, too low industrial output) which does not always have to deal with building new farms/mines/factories. Hmm...

Well, you could just let the player give subsidy/settings to the groups of specialized planets.

Consider a system like in Rome2/Attila. You assign governors to regions and can also pass edicts in those games. Edicts are region-wide (in your example, they could affect all planets in a system, for instance). Edicts give some small bonus to one or more game features - increased happiness, increased tax income, more unit recruitment slots, cheaper production, better spies, increased scan range whatever. The possibilities are literally endless and depend mainly on your game design and balancing. You could counteract it for balance purposes as well - for instance, maybe a "Technocracy" edict gives a bonus to research on every planet in the system at the cost of reduced tax income (money is funneled into research grants).

to regions

I don't want regions in this game (it does not fit other mechanics). Only a planet and empire wide levels.