I think it would be cool to add another trade option, which would be peace period. It would work exactly the same as the map feature, where people wouldn't be able to attack each other for the specified period of time. The main reason why I'd like to see this feature is to add some more weight to any peace deals, because right now people aren't incentivized to make them, which loses out on the diplomatic aspect of the game in my opinion.
I know backstabbing is something players should be aware of, and there is the popularity penalty for declaring too many wars, but honestly, trade embargoes from small nations are hardly a setback, even declarations of war from the AI are nothing serious, unless maybe in the earliest stages of the game.
The time I see this feature useful would be in the early-to-mid game, where players can broker a peace deal in exchange for provinces/resources, which would have to be enforced. As for why someone would want to make a peace deal instead of fighting it out to the end, in the cases where there can't be a clear winner unless both sides exhaust each other's armies completely. I believe roleplaying games would also see some benefit, too, although people there tend to respect peace deals more than in normal games.