A system would be great as long as they don't induce disruptive plays, so I try to think of achievements that are contributive to their team.
Prison Break
As an infected human, break free from resin nest and kill the Queen.
Queenslayer
As a xeno, kill the queen of another hive.
Sneaky Assassin
Kill the Queen when she is ovi'd.
Tall Hosts Bane
As a xeno, kill 20 humans.
Tall Hosts Devourer
As a xeno, devour 10 humans who are uninfected and have not been devoured before.
Small Hosts Devourer
As a xeno, devour half of the small hosts which are uninfected and have not been devoured before.
Sole Survivor
Be the only human evac'd via pod.
Fury
As tank crew, destory another tank.
The Return of the Queen
Being killed as Queen, evolve back to Queen and win the round.
Let There Be Light
As a survivor, repair power and switch on the floorlights.
Firefighter
As a human, put out 5 fires on fellow human with extinguisher or help intent.
Thicc Walls
As Hivelord, build 300 tiles of thick resin walls.
No Hive For Old Xeno
Win the round as the sole surving roundstart xeno.
Minesweeper
As a xeno, slash or trigger 8 mines.