Hmm. Don't know that it needs another, but if I were in the design department at Bungie... An aquatic race. Something like space-faring squids or jellyfish, that can move wicked fast in aquatic environments, and have to be fought in that territory. They'd hunt in packs, moving in three dimensions around you, wicked hard to defend against, preferring close quarters weapons (as projectiles work poorly underwater). Let's call them the Lurkers. They'll be a race that infests planets across the galaxy, the top of the underwater food chain.