Adding sound would be an interesting feature.
For example knocking down a guard with ranged attack should alert the guards nearby, because his body will fall on the ground and produce loud sound. Melee knockdowns should not, since the player can put him down slowly.

This will also make it impossible to fail the second tutorial by shooting at the guard that's supposed to see the other guard's body.

But on the other side, doing this will make ranged attacks harder to use, so maybe it should be classified as Hard mode.

Edit 1: Maybe set drag distance to 1.05, because dragging is somehow hard to do. It doesn't always work.