Here are some of my ideas for in game settlements.
Tariffs: if you sell your resources to another settlement, you will have to pay a tax to the settlement.
Trade deals: if 2 settlements want to trade freely, establish a pipeline, or work together on anything related to economics they have to sign trade deals. Trade deals can only be signed between settlements that have embassies.
Federations: settlements can also join together to create a federation (like the us), which will make laws/set trade policies that apply to all settlements. Federations will be lead by a president, who is elected by the settlements. The president is supposed to work for the settlements that elected him, but he can choose to be corrupt. In game this would probably be a setting that the player chooses from a pop up menu. There would be 3 leadership options, democratic, slightly corrupt, and rich moneybag. If the president chooses an option you dont like, you can impeach him with a majority vote.