Maybe it would be possible to have some "assistance" system, players beeing able to "co-op" but only as muscle and guide. What I mean is that a player can invite a friend to co-op, but only the story/questline things work for the inviting player. Hence not having 2 games but the oportunity to help one and another.
Yeah like in Diablo 2.
But i can easily accept the decision they made because i loved DD1 and it din't have COOP either..
I'd rather have a good singleplayer than a mediocre single and multiplayer.
