We kind of already have classes, don't we? :P
-Soldier, Assault, Engineer.
I don't want to get special classes with special elements.
There was this killstreak thread..
Wouldn't it be cool if from every 10th kill you got a medikit pack, which would heal ONLY your teammates with 25 hp, even if they are dead. So if someone just lost his life, a teammate who has gained 10 kills in a row, could revive him to 25 hp, though he could also keep on healing if he had more medikits.
In my opinion, this would not be overpowered nor unclassy, but a good reward encouraging teamplay.
As we no longer have a pickaxe, its slot could be used for all kinds of special elements like this, as I'm sure there will be more of them in the future.
Possible looks for it?

You got it! Now throw it to the blue fellow.