Will the book provide advice on constructing a cosmology in addition to offering a cosmology? Also, how do various planes relate to one another -- I seem to recall a Monte Cook Design Dairy entry that discussed times when planes drew close together and moved apart. (Eberron uses a similar concept, which in turn is a very old concept. For example, the Celts of the British Isles believed there were times each year when the boundary between world and the Otherworld was thin.)
How do various races on the planes relate with one another? For example, the standard D&D cosmology has a hierarchy among devils, and some products such as Planescape have included the Blood War. Is there a discussion of how the different planar races act amongst themselves, different planar races, and mortal races such as humanity?
How do the realms of divine beings figure in the planar cosmology? Do you think they should generally be separate planes or part or another plane as in many D&D cosmologies? How do deities interact with planar beings, such as celestial and fiends, and what role do divine realms have on more mundane planes?
Is there a discussion of various devices, spells, and ways of travelling from plane to planes? Any advice for planar travellers? How common is planar travel, and how does the frequency or lack of it impact the attitudes of natives of one plane to those of another? (For example, I imagine beings in a celestial or infernal plane might be fairly used to planar visitors. It might be less so for people in the lands of the Diamond Throne, the Forgotten Realms, or Eberron)
What properties of planes differ from each other, and how does adding certain traits to a plane create a "feel" of it? Is there advice on how to create new planes in the book?
What are some of the hazards you see for planar travellers? Also, what are some of the things that FRPGs, especially D20 products, address planes that you like, dislike, or think can be improved on.
Many cultural belief systems and mythologies on Earth refer to a central point, which has been viewed as such things as the cosmic mountain, the World Tree, or other concepts? (As I recall, the 1st Editiion Manual of the Planes described the center of the Plane of Concordant Opposition as perhaps the center of the multiverse and appearing differently to different people.) Why did Planescape make a city the focal point of the D&D multiverse? How did this decision shape the development of the D&D cosmology? In retrospect, considerig the way the standard D&D cosmology has developed, is there anything you wish was done differently?
What authors or concepts have influenced your conception of different planes? How did they do so, and can you cite specific examples?
What are some of the ways you imagine game masters and players using Beyond Countless Doorways ? What are some of the books and supplements on planes that you have enjoyed working on or using in your own games? What qualities make for a good book on the planes, or make for a bad one? Can you cite some examples of things from various books that pleased you, puzzled you, or impressed you?