Game lacks a very simple mechanic to have an important relation with neigbours, its trade advantage. Trades in stock market or in agreements should have carriage cost(also carriage time if developers wants more realizm). Carriage cost should be a logaritmic function of amount of resources carried and distance travelled. For example from Greendland to France, transporting 1000 iron, pay %10 transportation cost, for 5000 iron pay %3 transportation cost etc. If you buy from near country, (Spain) you almost pay 0 transport cost, which makes Spain more important for France. This nerfs trade a little, thus starting money can be increased by %10 or so....
It's just that simple, and countries with close borders will be afraid of simple ambargos, will try to form dynamic alliences because of it. Even this is enough to make diplomacy more meaningful...
Other details to consider:
-Trade Harbor: Just like how harbors reduce embark time on military units, harbors can also reduce imaginary embark time of trade convoys, reducing the cost of transportation. For sure railroad too must reduce transport cost drastically. With this some countries may literally play merchant style(Britain), buying and selling from overseas...
-Impactful ambargos: Lets say you are northern russia, and you want to import material from sweden, from game mechanics the cheapest road is sweden --> finland --> home, yet if finland ambargos you, you have to pay more for longer transportation, (sweden ---> northern russia by sea or sweden --> livonia ---> russia ---> northern russia etc. )
-Tariffs: On the perivous example, sweden ---> finland ---> northern russia, finland may put tariffs for transported goods instead of ambargo, such that finland gains little amount of money because of strategical position
. Now just a small land piece connecting northern russia to sweden would be more valuable, giving provinces geographical importance in addition to military and production importance.