Hello, I am new to the forum. First of all congratulations for this excelent of a game!
Maybe empires of the same race could also be less likelly to go to war if they discover an alien species. Also there could be empires wich are less or more influenced by the species of other empires, there could even be empires more likelly to wage war against othe empires of the same species (for example there could be an empire that considers itself the rightfull heir of humanity´s goverment).