An actual animation for the scoped crossbow and maybe an ironsight for a new rifle that can handle some decent accuracy... okay. But not for the rest of the weapons. They are close-range, mid-range at most, even the SMG and the Combine Rifle.
And for those who say it wouldn't change gameplay... common, just watch a video of COD and then one of HL. Fights in Half-Life are designed to be extremely close-ranged and fast. Stopping to aim with an ironsight WOULD change the pace of the game significantly.
So yeah --an actual animation for the scoped crossbow and, who knows, maybe an accurate rifle with an ironsight for the very few long-range fights. Perfect. Just not for the rest of the weapons. Not because it would 'ruin the game', but because it would be superfluous --unless they changed how levels are, in which case they would have replaced the whole HL formula except for the in-game cinematics. If they just keep the same type of levels, ironsights just wouldn't be used.