User blog comment:James12708/Reapers VS. Forerunners VS. Angels from Darksiders VS. Humanity from Halo VS. Council and humanity from Mass Effect VS. Imperium of Man/@comment-4721065-20130112104250

Given that you guys have introduced two sides from one universe (Halo's UNSC and Forerunners), I guess I wouldn't break too much if I say that IoM redirects some of their enemies into this: Tyranids, Orks and Chaos. All of those three can beat IoM ships in space-based combat if everything is even, so they wouldn't get offed too soon by the Forerunners and once they get to the ground, Orks multiply extremely fast via spores (apparently no planet has ever been ridden of Orks without all other life being purged from it) and can build tech out of the worst scrap, while Tyranids are in many ways Flood 2.0, if not better.

The most important faction here, however, is Chaos. They permanently reside in the Warp, a psychic realm that Forerunners would likely be completely unable to access, as they have no psychics of their own. More importantly, however, the Chaos Gods would be eager to find this new prey and so would try to twist and corrupt any Forerunners they find. If they succeed, it would be another civil war, and one where Chaos could never be defeated permanently.

This isn't as implausible as it sounds either, given that the Imperium has already succeeded in sending a mini-Tyranid Hive Fleet towards Orks once, it is quite likely they can do it again, and Orks don't care who they fight, so some of their clans can simply be bribed into temporarily cooperating with IoM. Chaos are somewhat similar in this too, and offer of enough guns/slaves will prompt sufficient quantity of them to head towards the Forerunners.