I read over a fair chunk of RedAce's post. So I figured I'd start my own with an exclusive Aliens perspective.
Unfortunately, I doubt there will be a single player campaign for the Aliens, so I'll describe how I think they should be in-game.
Campaign:
1.
BIG!- Parker said it best. The Giger Alien was at least a good foot taller than anyone there. Maybe not too much thicker in the midsection, but larger than your average bear..err human. I would assume the Cameron Aliens are more or less the same. The only one that would be smaller or equal height would be the "runner" dog alien. Of course it was rarely on its hind legs so it'd be safe to assume.
2.
Abilities- All the Aliens encountered in-game will have these abilities: walking on all surfaces, claws as their primary weapon, tails as a secondary, can either crawl or walk on hind legs(can't wall walk while bipedal), resistant to their own acid blood. They can break through grates, vents and some doors. A few can be almost instantly broken through, while others will take a long time, and the really heavy duty doors can't be broken through at all. They are equipped with biomechanical armor that somewhat protects it from small arms fire, and are excellent swimmers. They don't need air, but when swimming need to clear their "back tubes" of substances.
3.
Intelligent- The Aliens are very intelligent, more so with their "psychic" link to their Queen and each other. They can operate(destroy) some Human components. Examples: circuit boxes, control panels(protect them!), lights, etc. They won't always attack en masse from one direction. They will flank the squad if possible and try to simutaneously attack different directions at once. At the same time, if they capture a prisioner, they won't suddenly retreat, they will try to cover their breathren. They are also masters at stealth, hiding in any place possible, and moving unseen. While crawling, they make very little sound. As such, they employ ambushes whenever they can. However, if a battle is going badly, they will retreat.
4
Facehuggers and Chestburster- Facehuggers aren't super smart. They can wall-walk and leap a fair distance. But they only have one unwavering objective: To orally rape you. Again, like their full grown selves, they will use stealth to get you and the environment. Most likely to attack after a battle, or if you disturb(come near) their egg. If you shoot the egg, the facehugger will open it and try to escape, but not if its on fire. They can and will leap out of their eggs if your not careful(stupid stupid Kane). Not very tough, can be taken out with a single pistol shot.
Chestbursters on the other hand will never engage in combat, and once escaped from its unfortunate host will run and hide. It does take it 3-5 seconds to escape the body. Can't wall-walk, climb or do anything except jump and slither. Easy killed by a pistol or small fire-arm.
Multiplayer
No matter what class you choose once you start levelling up, you can "evolve" your model into a more efficient weapon after getting enough experience. All models (except Queen and Preatorien) can wall-walk and sprint. They can all: use their claws and (charge)tails and inner jaws, cannot be injured by their own blood, and can let out a screech/hiss which echoes through the level. Aside from scaring the hell out of human players, it shows your fellow Aliens your position and that of the target closest to you or in your crosshair. Aliens see same as AvP2: pheromones: Aliens red, humans blue.
Now when you evolve you will have the following abilities to pick from at various levels for all classes:
Internal Pressure- your inner jaws do more damage and once you die your body explodes scattering acid in a small area.
Hardening- your body grows thicker and harder allowing you to take more damage.
Natural Predator- your claws and tail do more damage.
Wounded Evolution- if your health drops below 40% your tail blade will fall off and you can shoot acid from it(charge for range)If you heal above 40% you lose it.
Chosen Evolution- permanetly change your tail from a blade to a acid sprayer.
Stalking Prey- increase your movement speed.
Venom Potentcy- increase your tail damage.
Environmental Adaption- can swim faster (and deeper to avoid motion trackers), don't take as much damage due to heat (flamethrowers included) or cold.
Class Abilities
Scout/Runner
Hive Growth- buids a piece of rapidly hardening resin that other aliens can hide in.
Terrian Sense- increased movement speed outdoors and in Hive levels
Acid Secretion Glands- allows you to fire an indirect glob of acid.
Skin Shed- your now smaller than the normal size, slight increase to attack, defense and speed.
Warrior
Pack Sense-increased damage when attacking with at least one other Alien.
Dangerous Creature- increased claw, tail damage and health boost.
Exoskeleton Hardening- your skin grows harder, giving you more armor.
Tail Whip- gives the abilty to send enemies flying when hit with a charged tail and possibly impale them.
Hive Sentinel- increases your movement speed and your pounce range.
Lethal Range- your pounce does more damage at range(cant use it close up) and possibly knocks your enemy down. (can still attack and look around, just not move for a few seconds)
Praetorian
Royal Guard- increases your armor, claw and tail damage
Hive Protector-slightly increases your movement speed and health
Queens Escort-when a Queen spawns and you stay near her, you recieve a bonus to all your stats.
Queen
Hive Ruler- Queen is on throne laying eggs, can break off. Play as facehugger or Warrior with base stats. Can play as Queen when under attack
Royal Highness- 2x damage and 30% damage boost aura around her. Can pick up enemies to headbite or throw. Tail can slice enemies in half.
Evolutional Matriarch- can move quicker, 2x attack speed. Damage taken reduced by half.
Survival Instinct- when health drops below 50%, Aliens around her go into a frenzy; getting a 40% increase to attack speed and damage.
I'll be adding more "Evolutions" later and be adding more Class specific ones in bit, slowly but sure getting there. As for how many, thinking about 2 each, depends on how many I get for each class. Enjoy guys

:blackalien: