I love how James Cameron's Alien(s), Starship Troopers Bugs (the movie, not the awesome book), the Zerg, and Warhammer 40k's Tyrannids all run together in one happy family. Obviously some of these races have relatively civilized, cognitive thought (Bugs, Zerg), and others are mindless machines of death perpetuated by their need to feed and expand (Aliens, Tyrannids) ... they are all organic reptilian/insect machines of flesh with the potential to rend Humans armed with Modern (and Future) technologies into tiny gobble bits.
Personally my favorites are the Zerg and the Tyrannid, if only due to familiarity. Personally I think every good sci-fi game nowadays has some sort of faction like the Zerg ... and Warhammer 40k has (currently) two! The Necrons and the Tyrannid. While the Necrons do not fit the specific distinction of previous races, they fulfill the same storyline element of a "pure evil" bad guy, who, at the end of the day, is the ultimate enemy of everyone. Now, perhaps in Starcraft2 the Xel'Naga will be the new "tyrannids" of the series ... however there is without a doubt that same evil presence within the Zerg faction as well.
Their threat is more feasible than the relatively fleshy Dinosaurs of the Jurassic Park series ... and when you come down to it, the "Zerg" are kind of like the baby of a Dragon Movie and a "Giant Ants" movie. While Protoss, Eldar, and Elves all fit their own role as those races that have lived far longer than humanity, and usually hold some dark secret, yet are the only ones with the knowledge of how to avert X,Y,Z disaster ... yet ultimately do not have the power to succeed by themselves, and thus need an ally of some sort (only when necessary).
Well, elemental has no "Ancient Forerunners (eldar/protoss/elves)" and has no "Monstrous SuperEnemy (Demonic Hoard, Zerg, Tyrannids, Undead Legion, Burning Legion)" ... or at least to my knowledge. While the playable races will only be Men and Fallen, I would certainly like to see elements of a Forerunner race and a Monster race woven into the story element. (Wait ... Halo copied from Starcraft!!! lol) For a game like Elemental, the Monster race would most likely be an "unstoppable" Undead Force led by titanic Daemons or what-not, while the Fore-runner race could simply be the titans themselves ... or rather relics and caretakers left behind by them.