Open borders as a tribute? Pretty simple. Often times I don't want to go to war with somebody, but I have to in order to get through their territory to attack somebody else. When you have a big army it's simply easier to crush them underfoot. However, it would be more realistic if I was able to demand open borders as a tribute. You could even make it so it's only temporary--lasting for as long as the peace treaty. Or whatever else. Just sometimes playing as the UK I get tired of having to crush the Ottomans just so I can move my army back from crusading in the Middle East to Counter-counter-Reform against the Spanish.