In order for this project to remain (fundamentally) an adventure game, I do believe we would have to restrict each player to a single main quest. The subquests, or diversions, would be integral to adding richness to the player’s experience of the world. They would also facilitate “party formation”, where a bunch of players join together to solve a subquest. However, to return to the original idea – for it to remain an adventure game, each player must have one single quest that is only accessible to that player. A single story that only that player has a part in.
You could implement a game ranking system, whereby say a player is awarded a gold star for each main quest completed and a silver star for each subquest completed. A money system would also be a good idea. The player could in true Quest for Glory style earn money by selling ingredients to herbalists or cleaning stables or working farms, etc.
I also should have added that everything (in my last post) applies to a single server, or Ultima Online-like shard. So that you could accomodate many players, by giving them access to different parallel, though basically identical, game worlds.