I'm a big fan of the favors system, it's a huge improvement on the previous call to war mechanic (where you could ally France and let them steamroll all your enemies again and again). However, there are a lot of times it seems where I have an ally who also allies my rival, preventing a DOW.
It made me think, maybe another option for favors would be to spend 20 favors to have an ally stay neutral in a war against a specified target. You could implement it so that it only works above a certain Trust threshold (either say 50 Trust minimum, or more Trust with you than the other ally). Or, it could become a Diplomatic option that is unlocked at a higher Tech level. Historically, there were cases where heads of state would negotiate neutrality in the event of a war.
It made me think, maybe another option for favors would be to spend 20 favors to have an ally stay neutral in a war against a specified target. You could implement it so that it only works above a certain Trust threshold (either say 50 Trust minimum, or more Trust with you than the other ally). Or, it could become a Diplomatic option that is unlocked at a higher Tech level. Historically, there were cases where heads of state would negotiate neutrality in the event of a war.