Imagine a Fate of Atlantis offering multiplayer experience in a coop fashioned way -- that would be absolutely awesome I think.
Which is exactly how multiplayer adventure games should be implemented if you ask me. Playing Portal 2's coop campaign only strengthened my belief. And there's been many indie adventure games released recently that featured more than one player character, or that gave the player character a partner to follow him around. Blackwell, Gemini Rue, Ben and Dan, Da New Guys... I'm not saying these particular series should go multiplayer, but anyone who's considering making a short freeware adventure game with more than one player character should seriously consider the opportunity of being the first or at least one of the first multiplayer adventure game out there because there's really just a tiny step to take to reach that level.
And contrary to the original poster's belief, there is a market for multiplayer adventure games, I never got the feel that we are some sort of single player adventure games only elitist community, we've had a lot of discussions about this in the past and there's a lot of people here who'd like to play such game. It just so happen that most people have been all talk and no action on that subject.
And like Scavenger said, MMO are not the solution. If you ignore the fact they suck balls at telling stories with the whole "there is no ending, everything is reseted every hour, nothing you ever do has any meaning, now go kill me ten wolves" gig, but adventure games aren't about combat, minigames and chatroom, they're about stories and solving puzzles, a successful multiplayer adventure game would be one where more than one player cooperate to solve puzzles and make story decisions, sometimes maybe even conflictual story decisions, why not?
Short game length
+ chatroom to throw ideas around
+ puzzle focused
+ linear, room/level structure
+ two player characters coop mode
+ timed and coop puzzles, maybe even character exclusive abilities puzzles
- obligatory Portal 2 plot rip off
- Quest For Glory overambitious MMO combat, AI stuff and minigames
= Maybe the first ever point and click multiplayer adventure game?