Oh yea, doesn't matter if it's a sequel or a remake of the original: a fresher version of Planescape would surely be much apreciated by a vast amount of people. I'm sure it would bring the necessary commercial success. There just are too many RPGs with "ordinary" settings on the market already. A game set in the Planescape universe would be a breath of fresh air for the genre. Maybe a remake with lots of nice additions is the way to go here. As stated abouve this would attract both old fans and new gamers.
I checked the link found above (http://www.city-of-doors.com/). Really not much material there. Lets hope those guys keep it up. (w00t)