I have never liked how powerful offensive sieges are how as a defender winning them only guarantees ur safety for 2 weeks how it doesnt cost much of the opposition other than resources so i suggest making it so that every nation participating in an offensive siege and starting an offensive siege leaves a gap in their armor so to say and sieging them gives u a 15% point buff against them as their forces are supposed to be concentrated else where.
So a 15% point buff in an attack on nations on the offensive.
This could make people think before attacking a nation as it would risk them getting occupied by a suprise siege.
I see a lot of attacks by nations to act as 'offense is the best defence ' but that is different from the real life situation where there is casualties in wars and weakens them to attack of neighbours.
This buff could even scale based on the size or warchest of the siege they are involved in.