Shroomy said:
I personally doubt that WoTC will ever license out the Planescape campaign setting. While WoTC has not resurrected the setting, they've made use of the materials in just about every book that has anything remotely to do with the Great Wheel Core cosmology. I can't see them giving up that sort of IP to an outside company.
It would be difficult to handle given that it was a metasetting more than anything else. You would either have to restrict the holder of the license to not including any references to all the other campaign settings that touched upon Planescape (often heavily), or you'd have to make them pay for the use of those other properties, or you would have to restrict them to just making a book on a heavily whitewashed Sigil and nothing else.
In the first case the setting would be either a hollow shell, or an outright mockery of its original self. Rowan Darkwood from Greyhawk but also involved in the fight against Zhengyi the Witch King in the Bloodstone lands of Toril? Can't mention it. An apprentice of Trobriand in Sigil? Can't mention that either. The ghetto of New Tyr in Sigil's Hive Ward? Nope because it's Dark Sun stuff. Tarholt the Dwarf? He's from Krynn so can't do that either.
The license wouldn't be worth it at that point quite honestly.
In the second case you'd risk having WotC standing over your shoulder to approve or deny everything you did, worried that you'd step on their toes with all the stuff that interacted with their planar material. And of course, since Dragonlance is already leased out, given the response the DL folks had to DL references in Fiendish Codex I, imagine if DL stuff popped up in a 3.x Planescape product line; wouldn't be pretty. And then there's the 3e FR cosmology, and you'd end up having to ignore it or somehow find a way to pay lip service to it in a 3.x Planescape line. Considering how often FR persons and gods were involved in things, you couldn't just ignore that material entirely.
Those two cosmological retcons make things difficult at the very least.
And for the third case, a book just on Sigil: it's possible, but we still have the issue of all the other campaign worlds who have people and places and gods etc showing up in Sigil, some as major things in the City of Doors or the city's history. Mention them and stay true to the material and you'll have the awkwardness of the 3e FR stuff, and quite possibly a pitchfork wielding mob of DL fans at the gates. Don't mention them and you've gutted a lot of what made the place unique and added to its own flavor. Now it's still possible to do a really good book, but you'd have to seriously mince words and allude rather than state certain facts about people, places, gods, organizations, etc. If FC:I could do what it did, you could take that sort of approach and do just a Sigil book, but it would be a challange.
That said, I'd love to see a hardcover on Sigil that was true to the (meta)setting.