Trying to devise some different ideas for diplomacy skills:

Aid relief: Diplomacy ship acts red-cross-like helping people from a planet being attacked, providing shelter, food and medicine.

Effects: reduce population razed per attack, increases population growth rate, increases relations. Should be able to target allied, owned and enemy planets. Could cost resources as well as AM.

Peace envoy recognition (passive): All empires recognize the envoy cruisers of the owner as peaceful vessels, thus destroying them is seen as a war crime.

Effects: The faction which destroys the envoy gets a negative relationship modifier against ALL empires. Should not effect owner if the envoy is scuttled.

Armistice: this TEC skill is diplomatic by birthright. could be on cruisers of a different race though.