Personally, I wish that alliances would reflect what they were really used for during the time period; attacking someone. In reality alliances were not about defense but were about attacking a mutual threat or enemy.

The French and the Scots allied not because they liked each other or to ensure peace between themselves. They allied against the English. Nothing in there was a "To help each other" concept outside of getting rid of the English threat.

I do wish that there was more of that in the game. It would still seem "Broken" to anyone in the lead as no one would want to be the "Ally" of the largest power around. Sure, they would want to stay friendly to avoid having you destroy them, but they would not ally with you against someone else.