I would finally like to see a good alliance system.
Magna Mundi Platinum did a good job when the player started expanding too much, the other nations around it got "frightened" and started to ally against the player.
The same could be done all the time. In reality, and that should be the scope of EU IV, there should be the try from all nations to install a balance of power somehow.
Imagine Spain grows too big. Then France, parts of Italy, and England (perhaps Portugal) will start to try to ally and to cut its power down- or try at least to cut its power down where it is not so strong, like when it occupied Holland.
The same with France. France was getting too big at some point, and the other countries around it allied, when it tried to invade Italy, the pope even got down to make an holy alliance against France so that it was not able to set too much of a foothold in northern Italy.
It also happened in the 30-year war in Germany. When the emperor became too powerful compared to the protestants, first of all, he had to defeat the german protestant alliance. After that happened, Denmark tried to invade Germany, and the emperor still was able to defeat them. Then, Sweden appeared, and after that, France tried to cut away its piece so that the Emperor had to give in and because of that, there was no hegemony of the catholics in Germany. Not much changed, except 30 years of war and destruction.
(this contrasts the badboy value where just everyone who was not in good terms with the player would declare war on him, no matter what its chances are to win the war; first, there should be an anti-player alliance; and when it becomes stronger than the player, it should declare war when the player expands too much!)
I would like to see these things to happen historically, and no player being able to conquer the world within a 100 years.
Even then, when a state overexpands, it will become much more decentralised and much more difficult to handle, like it happened to Poland-Lithuania and the Ottoman Empire.
All this, especially (three-way or even quadruple) alliances with a certain balance of power and a map where we are capable to see all alliances would help much more.