The problem of having a multiplayer feature in a quest driven game is that either the game has to work flawlessly as a single player too, or you'll end up having to wait around for your friends to be available, or you'll go ahead on your own anyway.
Of course you could make a multiplayer save, then play a singleplayer when your friend wasn't around, but not everyone enjoys playing the same game twice at the same time.
Imagine this being a very long and well done game, where it would take tens of hours to finish the main quest line, perhaps as much as 100h+ to do most of the side quests. Much like Skyrim (which has a pretty short main quest, but you get the point). I personally don't feel like playing several hours every day, but then on other days I'll play 10h+ easy. You'd have to live by a schedule or somehow work it out with your friend so that both of you want to play at the same time.
Making it seamless in such a manner that one of you can just leave whenever they like, and then pick up later on where you left the action, while the other one still gets to play as much as he or she wants, is pretty damn hard. If my Baldur's Gate sessions taught me anything it's exactly that. Also, I don't know about anyone else, but when I'm with a friend, I feel like I have to be rushing. It's really hard just to stand around looking at something or reading something when you've got a friend fidgeting next to you waiting for you to be ready.
Nah... Multiplayer is for FPS, RTS, or other such games you can pick up and drop any time you like, or MMOs with a persistent world. Co-op RPG quest line sounds nice and all, but it seriously doesn't work for many people. Still, doesn't Skyrim have a third party online thing up and running? Perhaps fans of 2077 can make the multiplayer component.