I think it would be nice to have an option to try to broker peace between two civs, just like we can now drag the entire world into war with alliance chains and joining joint wars. My last game I had an alliance with Jadwiga and Philip, but they were at war with each other. It would have been nice to have had the option to mediate a peace deal between them--and then sic them on Harald Hardrada, who was being a pain.