Aggressive Expansion and Its Problem

Aggressive expansion has only one purpose, to slow down the game. The problem here is that it is just a stop sign that gets smacked into the face of the player. It boils down to: ohhh your war was too easy? Now slow down pale you took already a huge chunk of land, at that speed you conquer the world in 100 years! But if you really want to conquer the world in 100 years even the small city state in India will fight you so you don't take Warsaw.

The challenge to slow down the player comes simply in an increased number of enemies you have to fight, with fewer allies that now hate you.

I think being a treat to the whole galaxy or world should result in forming a collation against you. But having a singular value called aggressive expansion forming this coalition is more than weak.

Possible Solutions

Aggressive Expansion should not result in an opinion malus but instead should more gradually influence the behavior stands. Imagine a small faction see your aggressive expansion but because of your long-standing good relations they don't believe they will be the next target and instead want to come even closer to you because of your strength they are not pushed away but instead pushed to you.

Some other nations should be able to be influenced via diplomacy to make them believe they will not be targeted but lied to instead. They felt safe but you will betray them.

Such interactions are rarely possible when aggressive expansion influences only opinion. In the real world aggressive nations are not university hated but many other nations around them become wary of their aggression and might act accordingly (trying to become more economic independent for example).