I was thinking and curious if anyone on this site has asked Monte Cook who happens to be my favorite game designer for an interview.... He and 3 other Planescape Vets recently released a planar supplement called Beyond Countless Doorways which while very Planescape in nature adds a few ideas that are non Planescape like.
For example Monte adds a concept of shifting planes that would affect planar travel based around whether they happen to be touching each other or disconnected at any given time. But the big meat of the products is information on a whole bunch of new planes and planar adventures which I guess would most likely be considered either demi planes or prime material worlds for purposes of "pure" Planescape Cosmology.
Kind of have a funny story about how I emailed back and form with Mr. Cook. I was at the time browsing Planescape auctions on Ebay to complete my collection and noticed a few auctions by this lady that in the description was offering customized autographs from Monte Cook. I thought it was rather odd that such an auction would not have it listed in the title so I thought she might be forging the autographs and not putting them in the title so ebay would not catch on.
So I emailed Mr. Cooks company and let them know about the possible forgeries expecting some sort of reply from one of his minions about checking it out and maybe a thank you. But I get an email from Monte himself thanking me for letting him know but it was in fact his wife cleaning out their basement selling some of their personal books!!!
They then offered to let me send them anything I wanted signed to them after I did not win any auctions. Which I have not gotten around to doing but I am thinking I may send my copy of Dead Gods or Faction War since they seem like cool adventures and I don’t use them much like I do the Plane walkers Handbook, Inner Planes Book, or Hell Bound. He also designed Tales of the Infinite Staircase, The Great Modron March, The Guide to the Astral Plane (which I actually use a lot) and Planescape Monstrous Compendium III.
As a follow up I just ordered the Beyound Countless Doorways book from my local gaming shop. I could have gotten it cheaper from Amazon but you really need to support local gaming shops when you can I believe.