I've always wanted this feature too but since it depends so largely on quests story and decision making it would be real incohesive? It's like Skyrim with co-op. It really wouldn't make too much sense. Even the battles and interactions with every NPC is a decision. Diablo 3 is a lackluster story so you can join and rush and tag along with whoever whenever. It's not the case here it would just ruin all that's precious with this game.
I have a brother that is unable to play on release but will a week later, so I plan to dabble a character solo and a different one with him and maybe another friend. That way I have some alone time to enjoy the game when I get off work early but also another game when I want to really co-op it up.
You never know what the future holds I just want the game to release and enjoy it now
