I think 3:16 is a wonderful game. Of course you can add more to it if you want and looking at the creatures I came up with a few more for Troopers to encounter. With Obsidian Portal now offering support and a number of you owning it as you donated to DriveThruRPG's Haiti appeal, there's really no excuse not to run a game, is there?
A group of colonists defying the authority of Terra. These could be criminals trying to outrun the law or settlers rejecting Terran life with it's enforced sterilisation and indolence. They may remind Troopers of family, hated rivals or even themselves.
All the inhabitants are identical; they may have a group mind or have radically different mindsets. They can tell each other apart without any problem but the Troopers won't at first. How do the Troopers deal with killing the same person over and over and over? Perhaps they remind the Troopers of someone?
These creatures have unlocked themselves from matter. They may appear as angelic figures of light, raging pillars of fire or invisible monsters revealed in the pulses of an energy rife. Give them a personality to match their current state of being and a completely alien outlook.
Horses, donkeys or zebra are the base creature. Herd animals working together for self-defence with a strong leader. They may have evolved to humanoid form or feral animals willing to kick Troopers to death over territory.
From mushroom men to roiling masses of slime mold. Sentience may be indicated by changing colours, rough speech or even telepathy. Their spores may cause hallucinations, be poisonous or infect the flesh of Troopers to turn them into zombie-like carriers.
The race may need another body to keep it alive or just to improve it's quality of life. Parasites may be from one or more races. Troopers who use a Weakness may be captured and risk infection and being Killed may actually mean being turned into a host…