I dunno... You'd have to plan out a completely different set of rules for how they evolve, move, and attack.
Here's one idea:
Since we have one race that has a technological motif (Humans) and another race with a biological motif (Aliens) we could have a race with a blend of both technological and biological features.
Like the Strogg. It would be fitting, since this is a Quake based game, to have a Strogg-style race. Besides, cyborg/zombie types appear in almost every other FPS series, like Doom, Quake, Half-Life, Halo, System Shock, and so forth. Let's call them the... Abominations. Because they're so hideous. The Aliens reject them because their biological impurity offends their instincts, and the humans reject them because they want to assimilate them. They tend to look similar to the Makron of Quake 4: A half-arthropodal, half-humanoid cyborg.
Note that the existing races so far have their upgrade system based on merit, for either building or killing. This can be spent on items from a stationary armory, or evolution from one caste to another. For the Abominations, merit is spent on either Shifting or Implants. The Shifting allows for a builder or fighter to change to different castes, each with a different set of implants available. The implants are likeable to weapons: These are used to attack. The castes are likeable to abilities: They allow an Abomination to, at the press of the C/crouch key (Hence why they are half-arthropodal: To give an excuse as to why they can't crouch) accomplish feats such as turning invisible (but not to built defenses, emphasizing the importance of good building), teleporting, and so on. To compensate for these most frightening powers, the Implants for each Caste will be nerfed according to the effectiveness of that caste's ability. Implants will have to be abotained at a "Material Processor" structure. However, Caste shifting can be done in similar manner to alien evolution.
Movement: Humans can walk and fly. Aliens can wallwalk and pounce. Abominations will have their movement tied to their caste. Weak castes will have invisibility, or teleportation, allowing them to move unseen. Strong castes will have to walk, like the Alien Tyrant, or Human BSuit. They might have healing auras, or radar, or something else to compensate.
Attacks: Aliens have to get up close to do damage. Humans can shoot stuff at range. Implants will need to achieve a middleground between ranged and melee. For inspiration, one Implant may allow for an energy blade to shoot out at an opponent, then retract quickly. These blades may be mounted on tentacles, to flail towards an enemy. Stuff like that.
This is only an outline of an idea, not a fully developed one, mind you. Note that the unnatural properties for this hypothetical Abomination race are also appropriate for a Demonic race.
