So here's a maybe a little outrages idea but pretty cool, How about Aria goes Power Hungry(And somewhat Ballsy) and decides she wants to extended the area of the Terminus System further into the Attican Traverse, of course this isn't going to sit well with the Council (Or the Alliance who are the one's who are like "
**** Treaties man! We're the motherfuckin' Alliance!" and are colonizing the area), But hey man, Lawless Pirates don't give a crap!
So Pirates starts attacking stuff in the Attican Traverse, mostly Human Colonies since the majority of Colonized planets are humans there, and so the Alliance respond in the most reasonable way they could, sending fleets and troops there to secure the colonies. But just as you think "Well This seems like quite a bad situation here, the Batarians(Who are like "Jeez we're still pretty ticked off at you Humans!") take this opportunity and side with the Terminus Forces, and begins attacking more Human Colonies in the Attican Traverse, which breaks out to a full on war, bringing in the Citadel Races (Minus a couple of races who are like "Yo' Calm down guys!", so basically no Quarians, Geth, Salarian, Elcor, Hanar, Volus. Unless they are Criminals on the side of the Terminus Faction)
So basically TL:DR Version
:
-Terminus Faction(TF) Expands to Attiican Traverse, attacks colonies
-Systems Alliance(SA) responds by sending forces to secure colonies
-TF Begins more serious attacks, Batarians joins (TF)
-Certain council races(Turians, Asari) joins SA
-Escalates into full on war, TF vs. SA
In Reality, any character can join this campaign, even if their race isn't involved in the war, they could act as independent mercenaries from the Council, or as Pirates/Mercenaries/Slavers, And i know this is a little out there, in terms of escalation, but it's just a thought