Well as a general rule, it's not possible to dismiss any noble from a position once it's been given to them (with the exception of marshals/vice-marshals/ambassadors). That's always been part of the game - you have to either protest them out or get the judge to ban them.
But what I think would be a good improvement would be to make it so that every duchy has a capital and units from RCs within that duchy can only be recruited at the ducal capital. You could also maybe change the distance from capital penalty so that it uses distance from the ducal capital rather than the realm capital.
That would give rulers an interesting choice to make. They could centralise the realm into one large duchy, which would limit how big it could grow, or create multiple duchies which would increase the maximum size of the realm and its ability to wage war, but with the trade-off of undermining central authority by creating powerful dukes who could declare independence any time they felt like it.
You could also potentially make it so that nobles of a duchy can only recruit at their ducal capital. That'd also strengthen duchy identity and would prevent realms from just creating a new duchy on the frontline of whichever war they happen to be fighting purely for faster refit times.
+1 I actually agree with this whole idea
