FYI I won't suggest blood coming out of your eye which then sticks on your vision
First a flinching effect, like in Half-Life 2: Episode 2, when you are hit the entire screen flashes white when you are hit,
It lasts like 0.5 seconds or so, but you'll flinch too when you get shot irl!
Muzzle flash, why this idea? Look at the header above and look at the blueman's rifle. Obviously it's put there for a reason.
Muzzle flash would indicate better when a gun is being shot. However the position of the muzzle flash will be tough to place for custom guns. Either 2 gun models have to be made, one without muzzleflash and one with. Or a voxel with the specified color must tell where the flare has to be positioned. Flare can be modded too.
Once you get hit, a transparent silhouette of a deuce will pop up on the leftabove corner on your screen, and the areas where you have been hit will glow red, like in Red Orchestra. Soldiers irl can feel a bullet entering that part of their body, however video games can't make simulate bullets entering our body yet, so that is a "compensation".
Ever thought about legshots? Once you get shot in the leg you can't jump/sprint/autoclimb and you'll crouch automatically for 10 seconds. Because taking a bullet to your knee hurts as duck.
tl;dr version:
dislike this topic if you didn't like ejected bullet casings, want to be slowed down once you get shot in da knee
and muzzle flash