Rezca
The Greatest Jaggi
I think it would make sense if war declaration would work a bit similar to peace treaties. If you declared war, you cannot undo it for a few turns (4 turns on standard speed?). Currently the only factor is the AI's willingness to negotiate, but a declaration should have real consequences. Sure, it's okay to let the AI back out a war quickly, just not immediately.
Maybe it can be based on relations as well. If they were bribed into a war or asked to help a friend. "Can you spare us this" is the same as the AI coming to you for help in their wars, but paying someone to is more a bribe than a plea for help.
Similarly, if they were asked into a war, they might try and negotiate peace sooner than later if they were just Cautious with the leader that asked them, and might stick around longer if they hated the AI that they went to war with.
They'd have more incentive to help in a war if they either or both liked the leader that asked them, and disliked the leader they went to war with. I'd imagine power ratings would have an influence with how quickly they make peace, but that's already implemented right?
As for the notes above, I don't know how practical all that'd be to implement - or if it'd even be worth doing so. They're just thoughts ^^