Kay, so here's my idea of how to make the most obvious fourth race, the hybrids (campaign-only, but fully functional) unique.Husk: basic melee unit. An infested, naked protoss, this fierce abomination has uncanny strength, and is able to leap across gaps and up cliffs. Succubus: troop demoralizer. A bizarre mix of terran and zerg genetics, this Azmodan-looking creature has no attack. Instead, it debuffs units. If a unit dies with the debuff, 40 damage is dealt in a small radius, and the debuff is added to all affected units. Chain reactions are possible. Vanguard: ranged saboteur. The vanguard can warp enemy armour and shields, negating them when damage is dealt to them. The basic ranged unit. Protoss-terran. Gestalt: psionic assassin. The Gestalt is a permanently cloaked (making it an excellent assassin) terran-protoss hybrid. Armed with a wicked psi blade and a modified gauss pistol, the Gestalt can teleport and assassinate any non-massive unit in one shot, as well as use the backlash ability, which deals 15 damage for every kill a unit has made. It can also use psionic ammo, allowing it to deal ten ranged attacks that deal 15 damage plus 25 to psionic targets. Shadow seer: scout and raider. The shadow seer is a zerg-protoss caster, based on captured high templar. It is capable of phase shift a building, making it invulnerable but unable to function, as well as use graviton shockwave, which launches all units into graviton prisons in a line. Its sight range increases as it sits still. It can use precognition to see everything a unit sees, and all orders issued to it, and it can merge into the twilight archon. Twilight archon: psionic saboteur. The twilight archon has a potent attack that spreads in a cone from the archon. It also has the ability to spread chaos, turn ally against ally. Lastly, it can use slowing warp, which works like irradiate, though instead of damaging opponents, it slows and lowers the armour of them.Progenitor units:
How does supply work? Instead of one building, we have two: the hive-mind emulator, and the fusion coil. The hive-mind emulator provides essence, which allows you to build more units. The fusion coil, on the other hand, provides power, much like the pylon. Instead of all buildings requiring pylon-power, fusion coils increase build-speed by 50%. If a building is within its power range, it will receive this bonus. A second fusion coil increases this bonus by another 50%. Fusion coils can become mobile, but move slowly.
As for unit production: cloning facilities. Each cloning lab, progenitor and halo starts out with one cell, which can produce one unit. The cloning lab, responsible for light hybrid units, can have an additional three cells attached, allowing four units to be built at once. The progenitor gets an extra one, given it produces the heavy ground hybrids, and the halo gets an additional two, producing the aerial hybrids.
Hybrid base defense are more unique as well. The xel'naga turret can switch, with a cooldown, between an anti-ground and anti-air attack. The stasis orb puts one unit into stasis (same kind of stasis as the old arbiter) for a certain amount of time, but deals no damage.
As for units (I've included terran-protoss and zerg-terran hybrids; they both exist in canon, I figured exploring the idea adds more variety to unit appearances):
Cloning vat units:
Brute: assault unit. The brute is a sturdy, powerful attacker, easily the strongest hybrid in terms of pure strength. It is a melee attacker, capable of charging into combat. All units hit by the charge are dealt significantly more damage than its base attack. Juggernaut (reaver in WoL): heavy tank. The hybrid juggernaut is a potent attacker, having vast reserves of life and being able to regenerate from death at a cost.Ascendant (destroyer in WoL): psionic master. The Ascendant, the highest ranking of all hybrids, is a potent spellcaster. Psionic assault deals ranged damage with splash. Gravity wells will destroy any massive unit with a 3 second charge-time beforehand. It can teleport foes around the battlefield, or knock back and stun all units around it. Psionic barrages will, after a 5 second charge-time, deal massive damage in an area (though much like psionic storms, it is avoidable).
Edited by Justin on 8/2/2012 8:43 AM PDT