What if you could choose what basic territory to give to that nation? Per se, as Turkey, you want to release Bulgaria, but you do not want them to gain Macedonia. When you click to release, a window will pop up showing what territories will be released, and you can click on provinces that will either be added/removed to the state when you click "Release" or "Play As".
It's a strange idea, but it allows players to liberate nations to their design (if they want to keep territory but release a nation to keep peace) (as so if, for example, Japan annexes Manchuria, but wants to release a puppet, save for a few provinces which will remain theirs, they can.)
(Player) Prussia invades Russia and annexes the two Polish regions.
Notices Poland is too expensive to keep in regards to revolt risk and ethnic issues
Decides to release Poland, but in current Vic2, releasing Poland gives them way too much land.
So with this feature, a window pops up with all the territory you can give Poland (their cores?) and then you can click the ones you want to give, so in this case, the player gives them control of the two regions they took from Russia, while leaving West Prussia and East Prussia, etc. under Prussian authority.