Brainstorming is fun.
1. the ability to burry into a human and control him.

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I know everyone thinks it sucks, but it has never been done and there seems to be a desire for.
2. Remote scout probes. For humans a little flying probe that can be remotly controlled.
Alien builder can glue some alien eye thingies onto the ceiling on and switch between them, to see if humans are coming.
3. Stuff to make enemies dizzy, stunned, blind, inverted controls, different gravity, pushed away.
4. Maybe a kind a teamwork concept. One alien who sprays something that works good against armor but nothing else. And another alien who sprays something that works good against health but nothing else.
5. Adrenaline mode. The more you get damaged, the faster you move and you do more damage.
6. Alien grenades that lie on the ground and jump towards the first human getting in range. Only explode on hitting a human.
7. Explosive slime blob. Charge and shoot, it sticks to surface/players and explodes after several seconds.
8. Teleportation belt. One time use, teleports you to surface your aiming at.
9. Handheld cloaking device. Blurs and transparent human, but disallows the use of heavier weapons.
10. Light armor - medium armor - heavy armor choices.
Goes from fastest-least protection-cheapest to slowest-most protection-costly. So players can fit a armor that best suites their playing style. Rely on speed take light armor, rather be a slow tank take heavy armor.
11. Personal shield. Protects against attacks and recharges. Goes permantly down if shieldpower fully drained.
12. Listener. Directional sound reciever. Allows you to hear the sound, as if you were standing 50 meters in front of you.