As if this hasn't been asked already: BOTS WITH ACTUAL SKILL.
Whenever we get someone to code AI.
Also: the Golden Gun from the book series (only obtainable with impulse 101)?
A gold Cougar Magnum? Wasn't in GE64, and would be as horribly unbalanced as the Gold PP7.
Stuff you can activate in maps to kill your opponents? (like in the movies)
Once again, wasn't really in GE64, but it's dependent upon the mapper. There are already things like the explosive barrels, computer monitors, and generators that follow this.
Backstabbing doing more damage? (or would that be too much like TF2 or inbalanced?
In GE64, shots to the back of the head were worth a lot more damage than to the front. The hunting knife/slappers wouldn't be the only weapons to have a "backstabbing" feature.
Awards for stealth?
Would be pretty hard to implement as there would have to be code added to calculate how "stealthy" you are. An achievement for using silenced weapons or something would be much easier and appropriate.