I apologize if this was already discussed or in the wrong forum, but I'm bringing it up anyways. And there are a lot of half-assed topics here so I thought I'd add to the half-assity.
What about the possibility of a Melee attack for up close and personal. I think it would be much better then running up in their face and trying to pull out your knife or just shooting randomly hoping to hit him.
1st Melee: disarming. If you got close enough to the point where they're pointing their barrel in your face you can hit the weapon out of their hand forcing them to pull out their secondary or finding another weapon giving you a chance to shoot them, stab them, etc.
2nd Mell: Rifle Butting, pistol whipping, or kicking. If you attack your opponent from the side; if you are unarmed, with a knife, hit the melee button and kick them in the face, knocking them down, giving you time to kill, if you're armed with a pistol, you whip them knocking them down, rifle; you rifle butt them in the temple, knocking them down, so on so forth. (That's a *i-need-a-broader-vocab* up sentence, in a hurry though.)
3rd Melee: Breaking knecks, taking hostage: this was inspired by splinter cell. You grab the character hold your knife to their throat and pull out your secondary weapon shooting your opponents or grab them then perform a neck breaking action button.
I'm thinking of other types of melee attacks you could perform too, so I'll be back tomorrow w/more.