A halflife mod called Firearms does something like this. everytime you get 10 kills or complete an objetive, winning the round, you go up a rank. nothing spectacular, just something to complement you.So there was a realod skill that shaved 25\% of your reload time off, there was another that made you a bit more accuracte, another one gave you medical skills, so forth.
Something like that ould be intresting. But the bonus shouldn't be too spectacular. Mabye an akimbo skill so you can use akimbo pistols?
Like i sed, if this idea is ever implemented, the skills must be worth getting, but musn't give the receiver an overpowering advantage.