I had an idea for diplomacy for this game, attached to this. Since races may randomly declare war one you even if you leave them alone and not directly anger them, they may as well escalate the war into total war and not want to talk to or make peace with you anymore, at least until a new leader takes hold, usually due to rebellions. Some of the atrocities that makes a race want to be more likely to have total war on you includes biological weapons on their planets or genocide of another race, especially their ally. Of course, multiple race can declare total war on you, but if one race make total war on you, this will not affect other races, who simply view it just as war.
Also, at the end, if you win the galactic council vote by 2/3 margin despite the fact that the other races voted for your rival or abstained, and you were at total war with your rival, and you accept the ruling, final war will ensure with a different message and ending. Basically, the other races do not find you fit to rule as supreme leader of the new republic and have as such branded together in a coalition led by the empire you are at total war with to crush your empire and defy the council's ruling. This includes all races that your are a good terms with as well as ones you are at war with.
Also, at the end, if you win the galactic council vote by 2/3 margin despite the fact that the other races voted for your rival or abstained, and you were at total war with your rival, and you accept the ruling, final war will ensure with a different message and ending. Basically, the other races do not find you fit to rule as supreme leader of the new republic and have as such branded together in a coalition led by the empire you are at total war with to crush your empire and defy the council's ruling. This includes all races that your are a good terms with as well as ones you are at war with.