I've heard this concept somewhere before. This is not completely my idea.
A slight problem with servers that have friendly fire enabled is that humans have a tendency to shoot eachother (obviously...). Some places have retrubution, which forces the person who teamkilled another to pay him back his credits/evos for the items the killed player lost, which wasn't always the greatest because sometimes you only do a couple of hp worth of damage and ended up losing quite a bit of cash, or some newbie ran in front of your fully charged luci with a chaingun-battpack combo and no armor (or possibly the "professionals" that shoot teammates at the medistation just for the hell of it).
A possible solution to these problems would be modifying the amount of damage friendly fire does for individual players to other people and to themselves, e.g. Newbie#2 has the most kills on his team, thinks he's big - so he starts shooting lesser players at the armory for amusement. I tell the server that I want Newbie#2 to take twice the damage he causes his teammates so when he shoots a teammate for fun, he does more damage to himself than he does to the teammate.
Things I would like to see if this were ever implemented:
g_admin !modff (player#) (w) (x) (y) (z)
w - Damage to teammates, eg !modff 0.5 (to make player do half the damage to teammates they normally would)
x - Damage to self, eg !modff 1 2 (to make player do normal damage to teammates but twice that damage to himself)
y - Damage to buildables, eg !modff 1 0 0 (to make player do damage to teammates but not buildings)
z - Damage to aliens other team, eg !modff 0.5 0 0 0.5 (nerfs player's damage dealing to 50% on all other players)