As I wrote above, " (except with new duke, but power-struggle between new duke and old lords would be understandable and i dont think we should prevent that..."
Of course there should be some struggle, but it shouldn't be that the Lords are somehow able to just force the Duke out of position. The Duke is supposed to be more powerfull than the Lords, not the other way around. A Lord who is unhappy already has an option, move to a different Duchy in a form of protest and thus lower the power and income of the Duke who they don't like.
Why would you want to ban people from taking estates? Thats how they join your realm/duchy! But, lords *can* kick out troublesome knights from estate.
Well if I think they are going to protest me out because they don't like me or whatever, I don't want to have them set foot in my Duchy. In fact, if as a Duke I don't like someone, I should be able to prevent him from every serving under my command, directly or indirectly.
And yes, Lords can kick them out and then they can take a new estate again in another region. So if you have one region which just builds !@#$loads of estates with a Lord who doesn't like the Duke then suddenly you have 10 people not liking him if he brings the clique over. And in turn the Duke can't get rid of the Lord, so can't stop this. That's why Dukes should be able to declare a Lord unfit and unwelcome in his Duchy as well. It will limit your power as Duke, but you can get rid of troublesome Lords. Otherwise they can just stack up knights and protest the !@#$ out of you, makes no sense that you can't get rid of them (not their title, but at least their region out of your Duchy).
You don't think we'd include the same restriction on protesting government members, that a protestor must be a member of the realm for 5 days before they can protest, for protesting dukes as well?
That would have to be a bare minimum, but if you see the above and Constantine's parts that is not near enough.