The little features in a game are what make it truly unique and stand out in the end, so I'd like to suggest uniforms and items/clothes and etc that get dirty slowly after you use them.
If I play a mission for an hour, and crawl 100m to get to a sniper position, my clothes sure as hell are gonna be dirty, show show that effect. The same would be neat on weapons. Especially with blood. In the options there has always been an option for blood intensity, so make that if your buddy gets shot right in front of you, and you get splattered with blood, it shows on your gun and body.
I am not requesting we turn Arma 3 into a gore/horror game, but these subtle effects would really add to the immersion, considering it's a milsim game.
I mean, just look at how much more awesome Red Orchestra 2 is just because when you get hit with a grenade your leg can blow off.