To get straight to the point, a nice feature would be some effect applied to an enemy when hit with a middle mouse bash.
I was thinking that the enemy being hit would have their view "pushed" away from the attacker, sort of to simulate having your weapon hit or being shoved backwards. Currently, going for a melee bash is suicide, and that makes sense (don't hit a guy with a gun), but in CQC the melee bash should be a last-ditch effort to survive, not just poking the enemy with your gun and hope he falls over.
To balance this out and prevent melee trolls, have the enemy being bashed still be able to turn back towards you and spray you down. This way, going for a melee bash is a highly risky move and a quick and prepared enemy can still kill you, but it gives you a chance at survival against the odds. I think this would work well with the knife, since the punch would "stagger" the enemy and you go for the kill with the knife.
Alternatively, just have the melee bash apply a shitload of suppression and gun sway, so a slow-firing weapon might be beaten by a fast opponent.