I miss planescape...
As others have said, considering how much IP WotC has already released in MotP, Web enhancement, Dragon Magazine and probably in the MM II that is supposed to focus on CR10-20 creatures, it's unlikely that WotC license this line.
Though a city book on Sigil by WotC seems possible.
Nonetheless considering that they said that books such as Guide to Hell and warriors of heavens got better sales than most planescape products, we probably won't see any books with the planescape trade dress.
Well, if I was at WotC here what I'll do (this might explain why I'm not at WotC

):
Considering that the MotP got the best score for a d20 product at ENworld, I'll think that their is a good customer base worth suporting.
Looking at the city book in the d20 market I'll see that every such books claims to be easily usable in any fantasy setting. Well, then I'll produce a product with personality that can't be easily inserted, but one that will allow the customer to insert anything in it (that way it doesn't require that the customer has already a homebrew setting to be used, nor will the customer with a published setting fear to buy the product as it won't alter his beloved setting).
Then I'll look into planescape and decide what will sell, then I'll decide to release a cities of doors line. Yes cities, independant books each on a single city. Things like MoF 192 pages fullcolor softcover.
First release, cities of doors: Dis
A plane wide city of devils! After all, Legion of hell is one of the most popular product, so this one must be a seller. You might allow devils with some human blood to allow friendship between some of those (and explain why they get a classes for people who don't like monster with levels of wizard), tiefling, half-fiend, human, and many other creatures that have landed here. The DM won't have to focus on evil things the PC might do(and they don't have to be evil, think ravenloft for an idea of the ambiance if they are good) and the consequences (a problem in many evil campaign) as a lot of critters here already do it, and do it better than the PCs, campaign could focus on dealing with creatures worst than devil in nature that lurk deep in the sewer. The sewer, the same places where portals to the abyss and anarchs hide, good adversary for cops-like adventure. With a plane wide dark city it would be easy to produce a survival campaign, and it might be a good resource for DM in SF setting who have planet-wide cities, or for those who like the cyberpunk genre.
Second release: cities of doors: the city of glass
The "Sigil of the Elements", an ancient city of Marid (who need to get their flying ability back), it will allow to introduce arabian terminology and see how customer might react to a 3rd edition Al-Qadim book.
It's a city within a glass dome on top of a giant iceberg in the plane of water, I'll list all the district and you'll see why it must work in 3rd edition.
Ale Distrcit, dwarves and gnomes inhabitant, does it need more information?
Alcazar, the Marids (and janns) district, they have no power in the city, it's a kind of noble district.
The Berg, merfolk place.
Central District, humans that deals in transportation and lodging.
Coral District, sea elves focusing on trading and entertainment.
The deep, kuo-toa district, their buisness is bounty hunting.
Icelake, inhabited by nixies, dealing with tourism and aquaculture.
Jeweler's district, an halfling district.
Drake's End, a whole district of half-dragons!!! they focus on law and government.
Reverie, the "entertainment" quarter, it's a district build on a cloud above Icelake, managed by cloud giants, its inhabitant includes nereids, sirine and others...
Sharktooth, the coliseum of the sahuagin (they deal in hunting too)
Wormtown, the magic shop quarter (a very good things in 3rd edition to have such a district in a city) managed by ormyrrs, a race of worm-like creatures.
Once customer have been tricked in this new lines of product, the third release: Cities of Doors Sigil.
This is the one that will perhaps need the most tweaking, depending on your tastes. Personaly I'll think the author would need to look in geography book on how european cities such as Paris or London evolved without order, you still find similar logic in those cities (though, you still can't find a straight street as opposed to the vast majority of fantasy cities, but that's another rant), and it would be better than claiming that you can find anything anywhere but that some trade secluded themselves in district (I don't see how temples could agglomerates with each other for exemple). I wouldn't mind if they forget to include the changes that faction war and die vecna die had on Sigil.
Otherwise, Sigil is the planescape setting, you'll get the cant the personality, prestige classes for every faction, and perhaps rules likes those in the planewalker handbook on how ideas can really affect things (perhaps using something similar to void point in Rokugan?).
Now, if WotC want to steal my idea, I ask that they send me every d20 products they'll do, up to the release of the Sigil book (that would be one, two or three year of free RPG product, I can dream

).