Prestige system?
Let me make something clear. Keep the "XP" out the Perk leveling system.
An independent Fame or Prestige system like in Torchlight, will be better. But what will it add without messing up the classes?
My idea:
100 Levels total.
Obviously it will take time to Level all the way to 100.
Gaining Points:
1 point for each Clot you kill
20 points for Fleshpound for each clot you kill.
x points for killing z ZED.
10 points for every time you heal a member you heal.
100 points for each time your team kills the Patriarch.
1 point every time you give 100GBP to a teammate.
10 points every time you give a weapon to a teammate.
5 points for every time you kill a ZED feeding on a teammate.
etc
(No points should be given for contribution to a kill, because it will mess things up.)
Bonuses
Small bonuses that do not add to any Class bonuses.
Examples:
Increase the amount of dosh you gain per kill by 10% every 10 levels (100% more dosh total).
Discount for non-Class weapons.
Any other bonuses the community can think off.
How The System Works In KF's Universe
You are a good survivor/warrior and so people know you and you can be recognized as such.
Of course, ranks can be added, but they should be KF themed.
I would also like the Rank of "ZED Food" for Level -100. This will be a special rank for cheaters or people who play pirated versions, and will come with the anti-bonuses of -100% money per kill, -5 blocks of weight, 50% less damage and 30% slower moving speed.
Now, Prestige/Fame can have an impact on changing your names color or something like an icon.
Why give bonuses? To encourage people to help the team and give to the over-achievers something new to do. There is also potential for a truckload of new achievements.