In EU3 we had an issue with cascading alliances, ie allies calling to arms allies that would call to arms allies etc. often resulting in large nonsensical wars involving half of europe. Thanks to paradox this was fixed in EU4 as junior partners in a war can't call their own allies.
But recently i was faced by a different yet similar issue that i call cascading war leaders, the issue being that the war leaders changes as bigger and bigger allies are being called. While this is not an issue while playing France or Austria, this become apparent when playing, say, italian minors.
Just now i was playing Milan, I declared war on the Papal States to take Romagna, the Pope was allied to Ragusa (basically bosnia+serbia) and Corsica. This should have been an easy war initially thought i could count on my eastern allies (namely poland and hungary), in case things would not go as planned.
1. Upon war declaration, Ragusa and Corsica automatically defend the papal states, fair enough.
2. Ragusa become war leader, call to arms austria, hungary, and some other minors, i'm short one ally which didn't even respond to my call to arms.
3. Austria become war leader, call to arms all its allies, including poland, and a bunch of other.
4. I'm left alone vs my initial ennemies, their allies, and the allies of their allies, and my allies. Obviously i didn't stand a chance.
I'm fine with Ragusa becoming War leader, it was significantly more powerful than the papal state and as its ally it was perfectly understandable it would protect them, but for me austria becoming war leader was too much, and the fact my own allies didn't answer my call to arms before the war leader changed twice.
So basically what would be great and less frustration-inducing would be 2 simple things :
1. Make it so the war leader can only change once. I've seen sometimes the war leader changing 3 times with each war leader calling their allies in turn.
2. Warn the player on the war declaration screen if the war leader is going to change upon war declaration because for now it's not very clear on what condition this happens and seems quite random.