I think the game needs improvement to the campaign AI first and foremost, and then any depth to the diplomacy system / things to do when NOT at war would be a welcome breath of fresh air. As it's been mentioned already, the pace goes too quickly from a complete nobody to high tier vassal/king conquering everything left and right with nothing but wars in between.
Better voting systems (including ways to influence the votes, be it by bribing with money, promises of future assistance, giving away a fief, giving away influence points, arranging a marriage), better politics, intrigue... so much could be done and expanded upon the existing skin and bones frame...