So here's my idea; characters live in a serene valley town, surrounded on all sides by impassable terrain. Life is simple there, peaceful, its only troubles typically being those experienced by any other rural community.
But the valley holds a deep secret -- it's the crossroads between many different worlds, and portals to these other worlds are hidden everywhere. Sometimes animals disappear; cattle and the like, but now people are also vanishing, accidentally wandering into hidden realms and finding themselves unable to return.
That's where you come in. Simple farmers and hunters, builders and teachers, even witches living on the fringes of society, banding together to track these portals down and get your friends and family back, common folk risking life and limb to traverse strange and often dangerous dimensions to return the lost to their former lives.
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So what do you think? Adventures would typically begin with the characters preparing for the expedition -- with the players having complete narrative control -- and once out of the valley I'll take a more active role in describing the dimension they've gone to as a more typical RP would go. However; I think it would be a good idea if the players themselves would also make their own alternate-worlds for exploration, then they could take over as game-master for that adventure.
Interested? Any good ideas to add?