This type of game option would be good, if by co-op you mean something like a PvE. I'd love to see all the players working together to defeat a common enemy, I enjoy these type of games the most and it would make this game a whole lot better. Some people might disagree and say that this can't be programed into this type of a game, but I disagree with these people.
I agree, it would be pretty incredible to take down some bosses along with a buddy or two. Just need enough people requesting it, and I'm sure they'd add it to the game.
It would be really nice to be able to clear some kind of dungeon with the help of some friends. It would boost your friendship, being a nice team-building activity, and also be a badass game idea.
While that would be nice, that would change the entire structure of the game...that is way more than a simple update, you are asking for a completely different game. You're asking for the developers to make a separate PvE game and combine these two games into one. I feel like this would require a LOT of time, money, and effort. The developers are better off creating a new game in that case, in my opinion.
I realize what you're saying but the game they are making right now is entering a market where it is facing at least 4 juggernauts in LoL, DOTA, HoN and HotS. I think that something to differentiate themselves would be very beneficial.