So, say you have this game all about questing, finding quests and completing quests. Many of these quests have some sort of story element that binds them to the world, not just "Go kill N rabbits and bring me their hides!".
As such, a quest could change things in the world for the player that completes it, like opening up new areas or changing how the NPCs react to you, so on. Many of these can be handled with tracking which quests the player has completed and reacting accordingly, but what of opening new areas, or cases where the change caused by beating the quest is something that affects everyone?
Players can't go around beating all the quests and simply leaving the world essentially "solved", and you can only mask others progress so much for new players. So what are some ideas for ensuring new players aren't met with a world with few unsolved quests?
One idea I had was to create circular quests. One quest will open new areas, but also unlock the counter-quest which will essentially reset the first quest's effects. It should be a simple matter to tie this counter-quest into the story (bad guy quest, perhaps?), and essentially making a revolving quest with one or more states that it cycles through.
With that sort of setup, new players will always have the full set of quests available to them, but they may not always be the same. The story would get kinda weird for those who stick around, but I'm seeing this as a more casual type game, with short sessions. The story elements are simply there to add value and meaning, and to motivate players towards new quests.