Two more ideas. Since the game will have missions, probably it will be more-lengthy than it is now. So, some enemies to spice that extra time...
a) Serpentine. A more-than-one-square-sized alien, it has four claws, a serpentine body with a strong tail and a mouth able to spit acid. A phenomenum unique to the Aliens, it's a Deformed that evolved considerably well through mutation.
It's not that fast, but characters without Fitness will have a hard time against it. It has three main parts: Head, Body and Tail. The Head is extremely high armor, attacks through the mouth, that is not strong, but can do lots of pain through chemicals. It can also spit acid, and it's immune to it.
The Body attacks with the four claws, doing 1-4 attacks each time. Alone, they aren't that dangerous but it's a potential threat if you are unlucky.
The Tail attacks lashing. It's a solid blow which can throw you away, giving to the Serpentine time to go spitting acid in you while it gets closer. The Tail is not that resistant, but destroying it doesn't kill the alien.
To make things worse, the Serpentine can with it's massive body destroy doors through charging. Her weak spots are the absence of vision(it senses based in proximity and feeling, as the Drones do. With a large body and charging, can pinpoint her enemies) and the Body.
b) Nash. Come on, you didn't really thought he would be dead, did you?
While you just woke up and found out yourself alone, Nash has being transformed into a special drone to the Queen. The Queen controls him, and she can use some of his brain's information to make Nash a skilled opponent.
First, he can use with variable expertise any weapon of the base. He has the same limitation as you do(ammo, reloading, aiming, distance...) but carry less ammo, in a result of his tranformation in a part-alien, he carry proportionally less weight. Second, Nash is not attacked by aliens and can command them to a certain extent(attack, run, separate, wait...). Third, Nash has natural regeneration, being able to run and recover in security. Fourth, he can attack at close-range without weapons, transforming his left hand in a claw. In this state he can't use most of the human weaponry available. Fifth and most important, Nash, as being a part-alien, has initially more resistance and speed than you have.
Fortunately, there's still hope.
Nash hasn't being completely transformed, and so can get control of himself once in a while, resisting to the Queen. In these moments, he won't attack you, usually leaving something and running. This can be achieved also when he's with short life, his will to survive adding to the will of not being controlled. If there's a permanent cure? That, you will have to find out...
Also, he can't use Medikits, his different physiology makes it to not work.