i would love a buy option.
after all there were event were provinces were bought from other countries, where a country wanted to buy first and not the other one to sell.
It could been an improved buy options.
You could "buy" it with ulimatum for no ducats, after accepting it would give 20years truce with that country.
You could buy for something you offer, you could either offer other province, +gold, etc.
why is it good? lets say you have an ally, you dont want break the alliance and conquer but they hold a province what belongs to your culture group (more tax, production, less unrest) and you have one province aswell which belongs to their culture group.
a succesfull nonultimatumstyle buy could generate 10 years truce aswell, or new diplomatic move, guaranted peace.
guaranted peace could boost relation without the cost of diplo relations inwhich you guarantee that you will not attack that country for 10 years.