Since the announcement of 4e and the revelation of the decision to drop the Great Wheel cosmology I have been wondering what makes the Planescape campaign setting special, and came up with several questions for the people of Planewalker.com.
- Does it require the Great Wheel cosmology? Or could a cosmology like the one in the Myth Adventures series work?
- Does it require the idea that the material planes are not special?
- Does it require that angels, demons, devils, and other planer beings be seen as (above) average Joes?
The following is all my opinion:
It would not require the Great Wheel cosmology if it was just getting started, as any cosmology complex enough to fit in the infinite would work. Now that Planescape has already existed and matured for a while, there is no going back. You don't just take Lord of the Rings and set it in the Forgotten Realms or a new world. It has existed for too long and has developed a life of its own. Too late to make such dramatic changes and pretend that they were there from the start.
It would require the Material Plane to be less special than the Outer Planes, as it does not by definition contain many of the peculiarities that define the Planescape setting - importance of belief, the nature of infinity, the reality of paradox, etc. However, you would still definitely need the Material Plane, not only to satisfy the first point and retain the nature of the setting (by being the source of much of the blind belief), but also to create a more familar grounded space to which Players can relate, even if their characters cannot.
It actually requires such exemplars to be seen as average Joes. You go into Sigil and rub shoulders with Solars and Balors. Sure, on their home turf they'll probably fry you for some real or perceived fault, but you're likely than not to meet them away from home and in some state of vulnerability. That doesn't mean the average Joes (humanoids) are as powerful as these creatures, but they have a greater chance to give 'em the laugh than their material planar counterparts, not to mention more likely to try.