Really nice ideas - I like them a lot. However, I would add this one thing.Does anyone know if changing culture after conquest is planned / announced by TW?
For me it's such an important feature in a sandbox. And it's not just that, current features like rebellions, loyalty or recruitment very much rely on culture, so if changing culture becomes a thing these features need to be balanced around it plus additional mechanics. For me it would be one of the top priorities.
Generally, if you conquer a settlement there should be a way to change culture over time. This process could be accelerated with building projects, perks and skills, governors and policies. Until conversion is finished, you face a higher risk of rebellions because people don't identify themselves with their leaders (lower loyalty). The other obvious effect would be the ability to recruit troops of your own culture.
You could also add all sorts of other side effects, for example that your kingdom policies are more efficient when the settlement has your culture.
If party speed penalties get applied when you are in enemy / neutral territory this should be affected by culture as well:
Example: You conquer a settlement you get -20 speed penalty in that territory because it's not your culture. This also means that the former owner of the settlement has an advantage in these lands until culture changes and the speed penalty no longer applies to you.
I think it should also be possible to play completely different though. Maybe you don't want to assimilate but prefer cultural diversity. This could also have benefits like a larger pool of troop types. If you allow people to keep their cultural identity it could also mean they rebel less. At the same time it might be a better target for the former owner because they already share their culture.
Or you mix both, assimilate settlements at your border but allow people to remain their culture in a settlement far away.
Obviously it's not realistic that culture of a settlement changes within a few years, but for gameplay reasons the whole campaign speed is already increased anyway so I guess it would fit the pace of the game.
Let's say you want to keep the culture of your conquered town, but also want to recruit certain types of troops from that settlement... Why not have the ability to invite a notable of a certain culture to take residence in your town so you can recruit troops of their culture? No culture change needed and you can still have (some) of the culturally specific troops you want from that settlement.