A very interesting idea
One thing I was thinking about was having quest related items/NPCs etc that could be specific to a quest some one is doing. For example, if the quest is to talk to a King in a castle, when someone NOT on the quest goes there, they see an empty room whereas a quest-er sees the King sitting in the throne and can interact.
This could be taken further, having a building or bridge that's destroyed and needs to be rebuilt. Once the quest is complete, the building and/or bridge is there to utilise.
Worth noting that I plan to have people be able to define a quest group (via dialogs with an NPC) so that you can operate as a group. I believe this was already mentioned in another thread too actually.