Shooters don't necessarily need more realism but they do need more depth. If you take the time to sneak up on someone then you absolutely should be able to kill them. Nothing pisses me off more than putting the time and effort into sneaking out of a surrounded compound in Planetside 2, flanking the enemy, and then dying before I kill more than one or two people(and that's with grenades) because two guys can subtract my health points faster than I can subtract theirs. This kind of system might give the new or bad player an advantage but at the expense of overly simplistic gameplay later on. You can learn the basics of survival in one or two rounds, after that nothing you can do or learn will ever make the game better. Instead, every player is funnelled into the same encounter every time, eliminating the need to think or try novel tactics or explore or exercise caution. Perpetual sameness. What fun!
A good start would be to change the way we handle damage models. Rather than forcing everyone to aim at the head, individual hit boxes for vital organs and limbs, and even their weapon, each attached to different consequences, would instantly add variety to every battle. If you could clip someone in the femoral artery as they ran behind cover they could still attack you but would soon bleed out, giving them a chance to respond and you a last ditch effort to kill them. A player with a gut shot could suffer some other effect while adding a reason for them to stumble over to the nearest medic. Limb shots could be employed in similar fashion while preventing you from raising a long-gun and aiming down your iron sights. Head shots could kill, heart and lung shots could let you keep running a little longer, only to slump over 10, 15 feet from where you'd been standing when you were hit, with maybe enough time to drop a grenade at your feet.
I don't like Call of Duty but there is one mechanic I like, that perk that allows you to continue fighting for a moment after you've been mortally wounded. Imagine how much more cautious you'd have to be if you had to creep around corners and cover obstacles to ensure the man you just shot actually went down rather than seeing your score pop up and a perfectly safe body lying there every time. Situational awareness would become almost as important as twitching.
We need to get rid of the boxes and hallways that dominate level design. The map should be more than simply the space in which the game happens. Providing multiple routes to a single area would allow you to sneak up on enemies or hide from ones who are hunting you. They would allow you to escape from surrounded spawns or bases to liberate your buddies. Getting slaughtered every time you run straight down the hallway into the enemy's base? Hang a right and slip past, then give some back or continue on to the objective.
Having said that, I do miss some of the oldschool Quake/UT style shoots. Arcade games have their place, too. Unfortunately, they've gone the way of realism, with every new game that has anything even resembling a gun homogenized into the same awkward, boring mixture of realism and unrealism, with titles losing their identity and what made them fun along the way.