A co-op mode for up to, say, four people might be nice. Let the host create multiplayer saves and such, and have a Respawn Vitals option which respawns dead quest-important NPC's if they get killed, and puts, for example, quest-important items back to their original location if someone throws them into the sea. Perhaps have two different options: Co-op, where everyone shares quest rewards and their location in the continuing story. And competitive, where everyone gets only what they get themselves. Someone who gets ahead of the others could kill off quest-vital NPC's to slow the others down(Maybe have them respawn after a while so it's not a permanent slowdown.). Sleeping could be done when everyone agrees on it, then the server gets shunted X hours ahead for everyone who's asleep. I think it would be very possible as long as people don't try for crazy stuff like 6+ player multiplayer games. Keep it nice and simple and it might work. The only thing that I really miss in a lot of modern RPG's is a co-op mode. That would really make up for a lot of flaws, in my mind.
