You always notice trends at E3. This year, I've noticed several in terms of shooter games, such as a lack of health and ammo packs, but co-op gameplay is becoming more and more prevalent.
It's epitomized by Army of Two. This third-person shooter is entirely co-op - either with an AI partner or with a human, split-screen or over the Internet.
Army of Two also brings in a concept that's more traditional in massively multiplayer online role-playing games. In those kind of games, you have the idea of aggro - creatures will go after targets based off how much threat they produce. In Army of Two, the bigger and flashier your gun, and the more you shoot at the enemies, the more aggro you get.
When you get ALL of the aggro - and you can tell how you're doing by a gauge at the top of the screen - you turn red and start to do double damage. It also means everyone is shooting at you ...
Why would you want that, other than the double damage? Because then your partner can be stealthy, turn mostly invisible and run around at a speed boost, killing people in melee combat. If either person gets too much aggro, they can feign death.
You customize your arsenal to help with things like that, both before the mission and at points during it. The guy who wants the aggro might put on different barrels or an underbarrel grenade launcher. The more covert guy would use a suppressor, or smaller weapons (such as submachine guns versus going for the mini-gun).
There's more than just that - shields, for instance, to reduce damage taken can be added to a gun, and some players might have a lot of "bling" added to their weapon (and show it off to other people by temporarily exchanging weapons in co-op play). Those aren't the only shields, though. You can take the door (or other part) off a car and use it as a shield, holding it up while your partner shoots over your head.
That's a co-op move. It's not the only one. You give your partner a lift up so he can climb onto things, and there will be targets that have to be co-op sniped or targeted -such as booby traps or fuel tanks. You can go back to back to cover all 360 degrees, too, if you need. Heavily armored enemies might need to be distracted until someone can shoot them from behind.
And they can drag their friend to safety if they take too much damage. It'll take a couple of minutes to recover, but you're back in the game if your partner keeps the enemies off of you.
There are a lot of high-tech gadgets for the discerning mercenary. You have various types of weapons - 30 plus of them - and each of them has five to nine parts that can be customized. Instead of a mini-map, you've got a global positioning system that'll show you where things are and what path you should take. But most of all, it's all about tons of firepower and using it well with your partner.