Right now you might think "Herp Derp, we already have coop, n00b, we r alredy play togeta!" - but that's no what cooperative gameplay means. As things are now we are just people who somehow happen to be shooting and slashing at the same enemies. We rarely if ever get the chance to actually work together on anything: This game has no way to heal each other, there is no "real" aggro from monsters, the area wouldn't support kiting even if we had aggro. All enemies are just as well killable alone and if you play together it's not because you need to, but "because". There still isn't a real reason to play together, except personal preference.
(Leaving Shadow Lairs out, it's another story.)

But I guess that's nothing new to most of you. At this point I however have to wonder why I didn't see anyone question this in about the one and a half year I'm playing. I mean, seriously, it can't be no one thought about this when playing with others?
Regardsless of all that, what I'm getting at is the following: The developers already have a lot of tools to make us play not just in the same level, but cooperativly with each other. And it wouldn't be to difficult either!

• Pressure plate - A pressure plate opens a nearby gate. A Heavy Statue must be placed on it to keep it pressed, and multiple pressure plates require all the Heavy Statues to be retrieved. A Knight may stand on the button to open the gate, but will not be able to proceed in the level until the Statue is retrieved, as there will be a Party Button on the other side of the gate.
• Alternating Switches - Often comes in groups. Hitting any one of these switches will change all of them. These are often used to raise and lower large numbers of gates simultaneously, or change the direction of retraction for a row of consecutive spiked floors. They are colored purple and yellow.
• Timed Switches - Hitting a Timed Switch causes something to happen that will reset after a short period of time. When the action has been reset, a knight may hit the switch again to activate the action again, and may repeatedly do so until the action has been completed. Timed switches are also purple and yellow.

With these, it should be a breeze to make people play dependant on each other. Finally let me spark your imagination with a few examples of what I mean.

• Two Timed Switches in different lanes which together open one gate but only stay activated long enough to get back to the gate. (Disclaimer: There would be a switch behind it for permanently opening the gate as to prevent leaving someone behind)
• A simple two player pressure Plate room
• Four pressure plates for four gates. Behind them are four timed switches which activate the center gate so the group can advance

... Excuse the crudeness but I don't get paid for this so it will have to suffice.

Say, would you like to see more puzzel content in general? Would you like to see what the team can make of this?
And most importantly: Do you even want to play as a team?

EDIT: I feel the need to mention that those rooms are just examples for cooperative gameplay. At no time did I say that they would be empty and that you don't have to fight in them. "Puzzel room" does not exclude the presence of enemies and a great deal of their appeal might even come from splitting forces and managing different objectives while trying to stay alive.