I really want a "divide spoils" feature. Like if you get called to war by an ally, and you have a claim to territory in the enemy's country, you should be able to press that claim as part of the peace. For example, I recently did a playthrough where I formed the custom kingdom of Deira (started as the Earl of Lancashire as a custom character, conquered Jorvik, gained independence from Northumbria, conquered Northumbria except Lothian which wound up as part of the Isles and then Alba). I married my heir to one of Alfred's daughters, gained that alliance. Alfred conquered East Anglia and formed England, then pressed his claim to Mercia. I had claims on Cheshire and Derby as part of the Petty Kingdom of Lancaster but I could only grab one by declaring war on Mercia at the same time as Alfred before he annexed the whole kingdom. Then I was stuck waiting for him to die (my assassination attempt failed) before I could grab Derby from his heir.
It would be super handy if, prior to all of this, I could have negotiated my entry into the conquest by saying, "yes, I want this and this and I'll help".
Of course, there really should be a mechanism for claiming foreign territory diplomatically anyway, and for relinquishing it (in another similar England game I had Lothian, but I wanted to give it to Alba for the Smooth Borders, and to do it I ended up using console commands since there was no way Alba was going to declare war on me).
