Instead of having kingdoms fight each other just have houses fight for teritory and make houses be able to ally others. When you ally another house you can freely walk trough their territory with the same bonusses and no cart stealing. This way you will have more factions fighting each other and much more drama and strategy imo. It will also reduce the whole world joining one kingdom and killing off the world in no time. Anyway i think you need a bigger map for it but imo it would be much more fun. Maybe limit the amount of allies you can have aswell to maybe 2 or even 1 idk. This is just a quick idea but could be expanded.
Most games dont have good allies functionality but i can see it working well in this one. You could even set up trade routes with your allies or let them get a percentage of your cart worth or whatever. Either way i think carts should give much more resources aswell so they will actually be worth fighting for.