Cooperative game play is important. I invested 100s of hours playing BG1, BG2, IWD1, and IWD2 cooperatively with 4-5 friends or family members. I may not have even discovered these games if cooperative play wasn't incorporated.
We always lamented that PS:T wasn't cooperative. I eventually played it, but delayed doing so due to the lack of cooperative play.
A simple way to have cooperative game play not impair the story is simply to have only one player play as the main person, and force the other players to play party members that are currently in the game by replacing the AI. As long as the party gets built up within a few hours, most would be fine just spectating. A good example of this set up is Secret of Mana. A player logs in, spectates until a party member joins (if one is not available), and then takes control of that character (walking, battle, tactics, not talking, not direct decision making of story related decisions, etc.).
For those of you that are claiming there are many multiplayer RPGs, you may be conflating a good story driven RPG for 1-6 players RPG with one of the many MMORPGs. There is a massive difference between the two types of games, and there are very few recent cooperative RPGs for 1-6 players that I'm aware of.
I've never played a mod in any single player RPG that was even slightly as good as the actual game, so I'd put that as a very very low priority.
