I really wish there was some kind of sensible province trade system, so I could evict a count from my Duchy and dump another duchy on him instead, so my demesne was more consolidated.
I'm not sure that would fly in a historic medieval hierarchy. Being rightful holder of a major title was a big deal, its not a bargaining chip to be traded away.
Correct. It would, however, have its uses in modeling the Russian rota system, where something like this in fact existed (when the Grand Prince of Kyiv dies, the next in the line of succession moves to Kyiv, his own successor takes up his previous position, and so forth, effectively leading to a "ladder-like" ascension through the hierarchy of titles). While there's less sign of it in vanilla (maybe the Byzantines?), it could also have its uses in mods (f'rex, a late-Han/early Warlord/Three Kingdoms mod that handles the buildup to the era, modeling cases where a person ruling one prefecture was reassigned to another one). This has not been implemented largely because coding for it is a huge pain in the tuchus for questionable benefit. That said, they did eventually figure out how to install tanistry, so...
EDIT: Ah, a missing period. Thankfully, it'll have less consequences than some other cases I can think of.