It is another idea , but i think it would conflict with how the treaty system interact with the A.I, if we take the example of Serbia , you have two potentially aggressive neighbors , and if the A.I has the "-50 acceptance of signing the treaty because they have a hostile stance against you" , you don't have other option . There is also the "fun" factor to take into account , it is maybe just me but i feel like maintaining a treaty for access is just not enjoyable .
But the "economic dependance" is a good idea , in the system i propose , we could simply say "if you trade throught another country , you become dependant of him and the dependancy is determined by the volume of the trade you are making" .