I must say, the things you really need in co-op oriented games is teamwork.
Make it so people are dependent on each other and discourage soloing (Unless it's for an objective) So that people actually stick together rather then getting them selves killed for the hundredth time in a row. I find the RPG system some mentioned to be nice, especially if you have different tech tree's so you can randomize and create your own soldier and ideal character for yourself instead of the standard class system, having the skills you want and find best suited for your play style, is a happy gamer. (Or dead one)
Also, maps should have tight spots and large area's, so that people can always use different strategies depending on their current situation, in tight spaces you will have to fight back to back to keep the enemy from advancing, ideal to defend each other but with little movement speed you can die pretty fast, whilst in large area's you are more likely to take cover and gun the enemy down cause of the large playing field, which makes flanking an ideal thing to do.
I heard someone say Republic Commando, I loved it and it really showed you how team work should be like during a fierce battle or just an infiltration mission, maybe you could get some inspirations from such games for level designs?
Also, the objective based game play makes Co-op games more challenging and you will need to know what everyone is capable of, really putting not only your trust but also your luck in the game to succeed, like having one person hack the door controls whilst the rest defend, or defend key locations to advance the level or suffer a time penalty, making it harder and in which you must make a tight plan which can only be done correctly with perfect team work.
Also, a VoIP in-game would be a handy addition, this way people can react to each other faster then having to type out every single word and suffer death by the hands of the long line of words, which can be frustrating sometimes.
The dragging wounded soldiers part sounds awesome and will always get my adrenaline packed, haven't seen much games doing that, being wounded and fighting till your death is always a good thing to see happening. Maybe add a survival mode? See which team can survive longer then the other for not only a competition but also just for fun, mostly the second for me.
Lastly, if you have enemies and I looked at the sharks, the 'deep wound' things would be fun, rendering the user from actually moving around much and having to depend on his fellow soldiers will make the game play even more realistic and actually builds a bond between the players, of course people will try to time rush the maps but you should do it with a difficulty system like in Left for Dead, the faster or better equipped (Ammo and Health) the team has the harder or easier it gets, but it never must get to easy or the fun will drop, but it shouldn't be all pre-generated in which you have a typical scenario, one team is full on health and ammo and gets like twenty enemies at them, the finish the fight with low health and ammo, team two however faces twenty enemies as well, however they get killed during the battle because they have no health and ammo at all, which is very frustating with some co-op games I played. However if you do it pre-generated you will have people that use tactics instead of fast trigger finger gaming, conserving ammo and health by using melee attacks. (If that is even possible)
Just some idea's.
For the love of the game! (And the joy of fighting alongside your fellow soldiers and of course the thrill of killing mutated sharks!)
~MCHurtubise