Here's another idea of mine. At a certain level, each perk get's a special ability. It can be level 1, 3, 5, any level. So the abilities for each perk:
Field Medic: Instant Heal
All players are instantly restored to full health and armor
Support Specialist: Resupply
All players are instantly restored to full ammo and their guns do two times more damage for a short period of time
Sharpshooter: Homing Bullet
All bullets fired by the sharpshooter will home to the nearest enemy's head for a short period of time
Commando: Identification
All players will be able to see enemies' health and stalkers from a certain distance for a short period of time
Berserker: Invincibility
The Berserker is invulnerable to damage for a short period of time
Firebug: Improved Fuel
The Firebug does two times more damage with the flamethrower and is invulnerable to fire for a short period of time
These abilities can be activiated with combos like kill streaks, etc. (which means that once a combo is done, the ability becomes available to the player for activation)
Kill streaks can include killing x number of specimens without taking damage, get x number of headshots within a certain time limit, etc.
All players should be notified if one player has used his ability
Not a very good idea, but I thought it would be worth suggesting
Field Medic: Instant Heal
All players are instantly restored to full health and armor
Support Specialist: Resupply
All players are instantly restored to full ammo and their guns do two times more damage for a short period of time
Sharpshooter: Homing Bullet
All bullets fired by the sharpshooter will home to the nearest enemy's head for a short period of time
Commando: Identification
All players will be able to see enemies' health and stalkers from a certain distance for a short period of time
Berserker: Invincibility
The Berserker is invulnerable to damage for a short period of time
Firebug: Improved Fuel
The Firebug does two times more damage with the flamethrower and is invulnerable to fire for a short period of time
These abilities can be activiated with combos like kill streaks, etc. (which means that once a combo is done, the ability becomes available to the player for activation)
Kill streaks can include killing x number of specimens without taking damage, get x number of headshots within a certain time limit, etc.
All players should be notified if one player has used his ability
Not a very good idea, but I thought it would be worth suggesting