Yet another long-winded post from me...
On the other hand, I suspect that the CRT is so popular because it's a tavern setting. The actual setting, I find is easier to play in because it's more relaxed, people actually hang out in bars and clubs in real life and thus have more of a feel for it. So I don't doubt that a tavern-like setting somewhere like Sigil would be just as popular, simply because it's a tavern. I'm a little skeptical as to whether the marketplace will be played in reguardless of where it's moved - afterall, there was little interaction with the actual market aspect.
Quoted for emphasis.
I think that this is dead-on accurate. It's a lot easier from a logical standpoint to explain characters meeting in a tavern or restaurant than it is in at what is essentially an open-air mall in modern terms. Many characters are unlikely to take leisurely breaks after acquiring things that they need, whereas at a tavern or restaurant based room would have that logical basis - everyone (aside from undead) needs to eat and drink sometime, and by default, most prefer to do so at a comfortable and relaxed pace. Making the redesigned Emporium into a more defined restaurant within a marketplace, however, may be a better route to go than the current open air environment with Feegee's stall.
Either way, I stand by my earlier post about the merits of using Sigil. Sites like
The Mimir and
Planewalker (which creates Planescape 3E Official material) provide plenty of online source material to draw from detailing the setting and culture of the City of Doors, and it would truly allow us to maintain the roleplaying potential that Juxta has provided. None of the other settings permit homebrew-world characters to visit as easily as Sigil would, and that in and of itself should be a strong selling point.
Might as well run down my opinions on the other setting candidates while I'm thinking of it since I am a setting student of sorts, and a tired and rambly one at that.
Forgotten Realms: Personally, I find Faerûn's (which, let's face it,
is the Forgotten Realms region most likely to be used considering that the others are largely if not entirely out of print) breed of high-fantasy to be a push in the wrong direction - mages are too prominent there for my tastes, making warriors (aside from a certain overly angsty drow-cliche-generator that is best left unnamed) feel somewhat obsolete. There's a devoted fan-base, but there's also a devoted anti-fan-base - not everyone likes the setting, and those that don't tend to be very much opposed to the setting's style (myself included in that group). I think Faerûn is responsible for much of the decline in certain character archetypes in many ways, simply because it is so over-exposed and over-prominent.
The other realms in the Realms have the distinct problem of cultural sensitivity, namely the already eliminated Al-Qadim's Psuedo-Arabic and Kara-Tur's Psuedo-Asian premises, for example, both of which run into potential stereotyping and political-correctness issues that could become problematic. As for Maztica, the Psuedo-Mayan setting module, that's well and good, but exploration of Maztica is limited even among natives of Toril, so how would we rationalize extra-planar travellers visiting?
However, the one type of location that I would actually like to see working in Abier-Toril would be an Underdark room where some of the morally-deficient characters that we have now would actually be
welcomed without looking like outcasts. A room where races that are known to be sinister could congregate and have a valid reason for not being persecuted on sight would be a bit of a draw I think. I'm sure that this would create its own issues, and that many players would be loathe to see a room made to allow proper evil characters to thrive, but I personally think that it could work.
Regardless of which region is used, if Abier-Toril is selected, I know that 90% of my characters would never go there due to their own opinions (including the rare character of mine born on Toril, of which there are... two

).
Eberron: Great setting concept, possibly the only flagship setting Wizards of the Coast still supports that I have any amount of appreciation for. While low-
level oriented, Eberron is anything but low-powered - one look at the Artificer class and what can be done with it should prove that in and of itself. I think the main issue with Eberron as a room would be the mage-technology and methods that magic are used in the setting, and how that would interact with the Greyhawk room. Fine setting, but I don't think that the sort of free-reign character creation that tends to occur in the ISRP could reasonably be brought here, and as has been mentioned, the devoted fan-base likely isn't there yet.
Homebrew: If Planescape isn't chosen, then this would have to be my second choice. I'm currently working on two self-created settings of my own (Veghinix, which is semi-standard high fantasy, and Risen Myths, which is futuristic and classicist blended together - both of which are in construction on my message boards linked in my sig) as it is, and have some experience in setting design. I'd be happy to help build up the core of the new project in any way possible to provide the type of location that could allow as much diversity in character population as possible. Anything needed, be it game-material related, artwork, or flavor text, I would be happy to volunteer some of my time to make the setting work. I think that aside from Sigil, this is the only setting option that would permit the same breadth of creativity that is currently allowed by Juxta.