Sorry about the capitalization issue. I'll try to keep this in mind.
I think this could still be subtle: buildings would look like they're merged, but they really are filling the multidimensional space that does exist and that we don't see. Maybe the idea of Sigil expanding is better. But I don't see any reason for the three ideas (Sigil expanded, merged buildings and multidimensional space filling) to coexist.
OK, I just got back from Vegas and lets just say I have a few ideas for the lower levels: Casinos, alcohol, cheesy shows, washed up stars, 2nd hand smoke and succubus EVERYWHERE!
Maybe the lower level of the Lower Ward, the heat from the Great Foundry and the smoke from the Lower Plane portals would recreate the Las Vegas weather nicely.
I heard Cirque Du Soliel was going to do a show reenacting the time the Lady of Pain went after Aoskar, but all the acrobats disappeared!
-420
Well, the lower levels of the Lower Ward (now renamed the Industrial Ward) is where all the sewage goes, and has been aptly renamed "Jubelix Village."
As for having areas of Sigil that are like Vegas, that could well in the Market Ward, since that gets the most 'out of town' traffic these days (while the seedier side can go in the Hive)
I partly like this; the Old City thing works nicely there. I'd also like to think that monuments like the Great Gymnasium would stay; from the bits later in the thread, it'd probably make a fairly spiffy house of Parliament. And the Shattered Temple should be utterly untouched.
The idea of a few old buildings at the feet of sparkly monoliths is one of those things that appeals to me; I always imagined modern Sigil as being a sort of Chiba City (the Neuromancer version, not the real one) meets New Crobuzon affair.
I'm somewhat hesitant about creating an UnderSigil when there's already UnderSigil, which - I think - has always been a hideously underused part of the setting. Of course, if this is what you meant all along, do feel free to ignore that.
As I recall, the city's always expanded all by its lonesome when necessary. But, that's not terribly interesting. I'd be partial to the idea of using Dabus and guildsmen - from something like a Linseman's Union, or such - to maintain infrastructure (ie, power, water, subway systems, whatever else) running through UnderSigil (and, y'know, keep whatever they find living there from eating it).
I wouldn't do that. PSCS UnderSigil is dangerous enough on its own, and I'd avoid saying 'Oh look, you've got a portal key, welcome to the Abyss' because it's really not cool in play.
Absolutely nothing. 
Wiring is cool. Wiring entwined with razorvine is cooler.
The expanded city also offers more space on the ground, which helps. Plus, the Taipei 101 building is only about 450 metres tall, so it mostly won't be a problem. Or, if there is a superstructure, could it count as a seventh ward?
Also, I kinda see the wards getting slangy names - PoliWard, CorpWard, and DustWard stand out at the moment as likely candidates.
An aside on this one: blades and neon make for such a cute cyberpunk feel.
A very late second on this one. You've always been able to look up and see more Sigil.
now there is a nice picture: owners of skyscrapers constantly building up higher to the point where they are touching so they fight to destroy each others skyscrapers to see who will go up further. and beyond that when they reach the midpoint fights against skyscrapers on the other side. I could see entire campaigns just about two competing skyscrapers with the pc's constantly trying to protect their skyscraper and destroy the opposing one wile coyote style.
Well let me say first that I am semi-new here. I've been browsing the forums and looking for areas that I can contribute to and since the FAQ (OMG I read the FAQ! 
) suggests the project forums I figured I'd drop in and offer my crafting services.
Those ranks in Craft: (BS) should finally pay off. Anyway I'm a fan of Planescape but I don't have much material from it, sadly boxed sets are highly flammable and theft-able. Nor did I get to play often. I am, however, a fan of D20 Modern and I have done some conversions and independent RPG creation and I'd like to contribute in any way I can.
That being said somewhere I have a draft of an 'arcane generator' that was in development for a friend of mine's sci-fi/fantasy rpg chat room. As soon as I find it I'll drop what I had and hope it is useful. Until then my services and time are at the project's disposal!
First up, welcome to PW LordNightwinter and the UPS project
Secondly, I have no ideas what to say! Erm...just have a look around the posts in the project. A good place to start is the Thread of Threads as its got links to the most 'important' bits of the project so far, from there, enojy the project and throw in anything you think would make the project even better !
Have fun
Kal
My suggestions? Hit the Planewalker.com Encyclopedia and start reading mate- theres ALOT of Planescape material to play with.
Holy shnikies man, that's a lot of articles to read. I have time on my hands, but not that much time! I'll start by reading up on sigil and thinking of contributions for the city. That's a much less lofty goal.
Damn. The place where I work has really horrid net connections. sorry for the double post.
Let me put it this way- PS is literally like a whole other history.
Empires crumble and rise, races are created and go extinct, and the Gods themselves are known to kick the bucket- to attempt to learn all that at once is impossible.
As you said: Pick one point, research it to the point where you know if upside down, sideways and backwards, THEN you can start understanding the big picture that is Planescape.
Even so, using Sigil as that starting block, you have ALOT to familiarize yourself with 
Alot.
A WHOOOOOOOLE lot.
Lol, get reading.
Empires crumble and rise, races are created and go extinct, and the Gods themselves are known to kick the bucket- to attempt to learn all that at once is impossible.
As you said: Pick one point, research it to the point where you know if upside down, sideways and backwards, THEN you can start understanding the big picture that is Planescape.
Even so, using Sigil as that starting block, you have ALOT to familiarize yourself with
Alot.
A WHOOOOOOOLE lot.
Lol, get reading.
Me: ROFL
Thanks for the dissertation. I've actually run PS campaigns in the past (though it's been a while). Since the introduction of 3e and 3.5 no one wants to go back to good old PS or DS. I have to purge the old records in my head every couple of years to make room for the random crap that work crams in. That being said I took a lot of 'liberties' with the setting, case in point the classic tiefling 'courtesan' I suppose, it's easy to be imaginative and you can get very inventive as well.
An interesting question.
What if the roofs of two opposite sides of Sigil will touch each other someday?
Lol, who says they arent already; in some fashion.
Actually, wgar, 2e Sigil has the same shape as 3e Sigil-a Torus (donut-shaped with inward-facing opening) with the Cage on the inside.
Also, Sigil tends to keep all the extra-dimensional stuff subtle so that a berk can always second-guess himself as to when it happens ("I could've sworn that street used to be longer"). The only time it gets obvious is for the Sod who just got mazed.
(ps: Capitalization really helps when you're making writing complex statements)