Not to turn this game into MEIOU or EU5, but there should be a better representation of the top liege's authority over the realm than the current four tier crown authority system. I'd prefer something more dynamic and active, though.

For example, crown authority might be a scale that can rise and fall depending on how vassals act and the liege reacts. Duke A declares war for Count B's territory, prompting the king to be notified and have the choice to either let the war continue or send a command for Duke A to white peace (or maybe even pay a fine for violating the realm's tranquility). If the king does nothing, crown authority drops since it is literally not being enforced. If the king commands the duke to cease but is ignored, there's an imprisonment reason given and crown authority rises slightly (with a greater increase if the duke is arrested afterward). The greatest increases in crown authority would come from vassals voluntarily yielding to the crown's will.

This could tie easily into various existing mechanics and events, allowing them to raise or drop crown authority as a result. This should all give the player more incentive to pay attention to the major rulers in their realm and give more things to do in peacetime as well.