Considering that the city is conveniently circular in shape, forming a bound space which a massive portal could occupy (as well as being innately connected to portals)...
Has this idea been brought up before? Surely it has, considering how much Sigil lends itself to the amusing thought that it's really one gigantic portal; probably with something really nasty like the far realms on the other side.
I think this is a really interesting idea, and it adds more potential depth to the purpose of Sigil as well as Her Serenity. Perhaps the Lady of Pain herself is the key to the portal? Maybe the portal leads to somewhere outside the multiverse, or maybe it leads into the multiverse? It might even lead to the "real world," but this sort of connection doesn't appeal to everyone. It may instead lead to something beyond the Far Realm, or the Macrocosm, or some other kind of reality. There really is only speculation, since I'm unaware of any canonical source other than the one already mentioned entertaining this idea. Was there anything concerning the result of the cube passing through the torus of Sigil? Also, are there any other canonical sources or even something from Mimir touching on this subject? Thanks!
That's my fix for the conflict between the Campaign Setting's idea that you travel to a random plane if you jump off Sigil's "edge" and A Player's Primer to the Outlands' idea that you just fall forever. Sigil is always a portal, but one of those portals whose key and destination continually changes with no perceivable pattern. Different keys may also open portals to different destinations. Thus, depending on what the person is thinking, doing, or carrying, someone jumping off the edge of Sigil might end up anywhere, or it may be that no portal activates and the jumper simply falls forever in the magic-null circle of the Land.
As always, the Lady of Pain has control over it, and can prevent undesirable things from entering if she chooses to do so. I assume the portal is almost always one-way, going out but not in.
My take on it is that, the ring of the city itself is used as a bounded space by the Lady as a shifting portal to the inner planes; which she uses to produce the various natural phenomena regularly observable within the Cage. For instance, it rains in Sigil, a lot. And yet, there's really not that much atmospheric space inside the torus for rain clouds to form, so all that water must be falling down from somewhere else. Just like everything else in the City of Doors, the weather itself is imported via a portal, albeit a giant one.
When the Lady wants rain, she uses the city itself to form a portal to the elemental plane of water at the center of the torus. Gravity pulls the water down in all directions simultaneously so it breaks apart and falls as rain across the entire city at once. When the Lower Ward's smog gets too thick, her Serenity opens a portal to the elemental plane of air and allows fresh air to blow in as cleansing wind. Likewise, daylight within the City of Doors is provided by a portal to the plane of radiance. The light doesn't appear to have any particular source to Cagers because they're all standing in the portal, the torus, itself through which the radiance is flowing. It's kind of like trying to identify the headwaters of a river while being fully immersed in that same river. Whether a person could conceivably cross-through the portal to the inner planes while it's in operation is up for individual DMs to decide; though I'd imagine that the Lady endeavours to keep the portal uni-directional.
The circles formed by either edge of the city on the other hand, are separate portals to some mysterious void which can be handled in many ways, much as Rip suggests. They're likewise uni-directional, but they only allow things to leave the city. They both serve the opposite purpose of the center portal; providing a means for waste material to be vacated out of the city. The Ditch, the city sewers and even the city's atmosphere all empty out of either side-portal to wherever the Lady chooses to send the city's waste. The fact that individuals can fling themselves out through these portals and go to any number of places or nowhere is an incidental side-effect that the Lady cares not one wit about.
So, in essence, the shape of the city itself (Unity of Rings) forms not one portal but three (Rule of Threes); the Spikeward portal, the Downwards portal (through which waste is vacated), and the Center portal (Center of All) from which daylight and various weather phenomena originate. That's just my take on it though.
I suspect that if the Lady's source of power is the Spire (or even just Her home), that She uses the city's Torus as the portal to and from the interior of the Spire! Nobody else can get there without the proper key, which only She knows. However, they CAN use the Torus portal to get to other locations (at random, because nobody knows the proper keys). If a poor sod doesn't have anything on him that works as a key, down the Spire he goes, since the portal doesn't open... this explains why some berks wind up on a random plane when they step off the edge of Sigil, while other poor sods just fall into infinity...
There was a story in SIGIS once about a cube from Aceron passing through Sigil's sky. I suppose it's possible, and it has even been implied elsewhere that the bounded spaces on either side of a portal need not match in size. I'm thinking of another story where a small "snake hole" was a portal, and the key was a broom handle, so a curious housewife accidently triggered it.