I remember seing in one DD (unless i imagined it) that you said you would try to make AI smarter to not get used by players to fight player's wars all the time. Plus one for this change.
There should also be some metrics of how much is the player involved in AI wars, ie. if player is just sitting and not helping AI should know, relations should drop and in worst case AI need to break the alliance. Metrics should be something like : number of battles joined from total number of battles, units killed and lost, AI is losing war - should be worse for the relation with a player if not helping, AI winning war should make AI forgiving for players non-involvement, distance - AI should not expect much help in trans-continental wars etc... These all could be built into Trust modifier, but it should be transparent to the player at the end of the war - like saying: "You gained/lost X trust towards AI due to: responding to call, joined battles, occupations, giving territory etc..".
agreed that it should be more dynamic, like mentioned with it being a ticking modifier instead and modified by giving claims and cores to AI, but also if the AI has any interest in this war in the line of giving cores and claims. Like having the player having a core (greater modifier) on the province AI has a core in which case it could be cool to implement a system where you could promise the AI this province. This could already be done by losing the core in order to have AI as ally, which would make me think more about losing a core to gain and ally to gain two other provinces. Having a claim should be a smaller modifier but still count towards their willingness to join.
Add the modifier "has taken core in ally war" where you take their core where you only has a claim but still have the modifier to a smaller degree when you have a core yourself.