I think now every Duke in France has the potential to win against others, no matter of the size. Take my action against Fas. I would have one with one third of my size as well, just because I was a vassal of Germany at that time and Fas refused to fight it. That could work as Anjou or Valois as well.
Still, I have no problem with being a Duke in another Kingdom, as long as it has a common border with France (Provence, Lower Lorraine, Upper Lorraine and Dauphine come to mind). But the goal should be to join France over the longterm.
And I think we should restrict pledging to other Kingdoms. Right now it's a real exploit (which I used myself, I know). Probably we shouldn't pledge to another Kingdom at all.
Also, we should have wars within the Kingdom. As we rehost very often anyway, we could just have one side DoW, then edit the save to get the liege out and have the Duke as vassal of France again, then rehost. This would make it more realistic, and I think correcter as well. Still the King could decide to step in and side with one of the Dukes (who'll give him something for it
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All these changes should make it more interesting to be a King. He wouldn't be only a puppet, but have around equal chances. He still could have problems if the other players decide to gang against him, but a Duke has that problem too. It's then just about being able to defend oneself millitary and especially politically.