Force Diplomacy mod is the answer to the diplomatic problem; though it can be considerably annoying having to do it every other turn for awhile if the a.i is bent on attacking. It lets you roleplay or create situations to your hearts content.

Lack of a aggression is still a problem on VH sometimes. For even decades some factions (Getai, Sweboz, Casse, Bactria, Parthia, Luso are the most likely for this) will just not do anything and I have no idea why. Yet sometimes they will randomly spring to action and start on a conquering spree.