Alrik,
Let us worry about how long things will take to implement
You'd be surprised how easy it is to add new MP modes, once you have a solid networking framework.
Personally I don't do much co-op stuff, mainly due to lack of friends that play games. But I can see the potential and I guess lots of people are into co-op. It is an extra selling point, so I guess if you are going to implement it, dont do it afterwards, but before release, at least from a selling point of view.
All that grieving and stuff is only when you play with unknown people, but you can limit it to friends etc. I dont know what kind of MP already was planned.
co-op campaign (with drop in ability) could be cool too. But then things do get harder to implement I guess. It is hard to predict if it will be fun with the few info I have about the game.
