Ever since the Alternity game was released, I've been wanting to do something substantial with the idea of a crossover between Star*Drive and Planescape-- there are a number of points where the settings' similarities or differences make for an interesting interaction.
Some of the biggest similarities are the focus on exploration, ideological conflicts, and the idea that the PCs might not always be on the same page, goalwise; even closely-allied PCs will generally have some conflicting political goals. The parallels between the Factions and the Stellar Nations (at least, how the Nations function in the Verge) really stand out to me.
The big, interesting differences include the difference between magic and technology, the overall "advancement" of government types, and the fact that in Star*Drive, temporal gains bolster ideological goals, while in Planescape, ideological gains bolster temporal goals. (Belief-as-power.)
Star*Drive, despite being mostly non-magical science fiction, has strong hints of interdimensional travel, from the main means of FTL transport to the near-deific technologies of the Precursors.
I'm wondering what other people think about the idea of some substantial crossover-- or even merging the two to form a third setting. (The Planescape side of things would probably resemble Urban Planescape quite a bit; seeing that project helped motivate me to post this.)
I've come up with a few ideas for points of examination (there are Star*Drive spoilers below):
** SPOILER SPACE **
The aforementioned parallels between the Factions and the Stellar Nations: There are a number of corporate and/or trading nations that would be sympathetic to the Fated, while the abolished Dreth Commonwealth and the thriving Thuldan Empire both have ideological similarities to the Godsmen. (Both nations seek self-perfection within this lifetime, as opposed to across numerous lifetimes.) The Galactic Concord often fills a role similar to either the Harmonium or the Trascendant Order. The Orlamu Theocracy's "Divine Conscious" could be similar to the Great Unknown of the Athar, or it could be a Power's muffled attempts to contact the sentients within the Star*Drive universe. The Athar could easily be an ally of the Stellar Nations against the I'krl-- whether the I'krl are real Powers or not.
Star*Drive takes a mostly technological approach to dimensional travel, which humanity is just scratching the surface of-- the method of FTL travel used by humanity involves an alternate dimension that's frighteningly close to the realm that the I'krl (a group of aliens banished from this dimension before humanity evolved) are imprisoned within.
There are numerous hints at magic within the Star*Drive setting, including a kind of technomagical device wielded by a species of spies and saboteurs (the Gardhyi) and the shamism of the primitive sesheyans which is often explained away as some form of psionic talent.
The issues of rapidly integrating modern technology (or even more advanced items) into Planar society seems to be a major focus of the Urban Planescape project-- but a merger with Star*Drive makes this even more complicated, due to the wildly divergent technologies wielded by many different species.
Several Star*Drive species are similar to, or possibly linked with, races from various D&D products which would of course be accessible in Planescape-- the Fraal (and Thaal) could be what the Oortlings were before they were corrupted, and T'sa seem at times quite similar to CG Kobolds. Gardhyi are awfully similar to Keepers. The Weren are much like an advanced Yeti or Sasquatch-- and other Alternity settings often declare them to be the same species.
One area I've given a lot of (sadly unproductive) thought to is how Sigil would change-- Sigil is supposedly the biggest city in the multiverse, and I can't imagine the Lady allowing it to lose this status. With the huge metropolises of the Class 1 worlds, and enormous planetary cities like Catalog or Aleer, Sigil would have to expand to become as large as a planet in order to accomodate that kind of population and commerce. I'm still not sure how to handle future-technology infrastructure or post-industrial commerce in Sigil, but I can see the city itself being a lot like Coruscant from Star Wars, except on the inside of an enormous torus-- possibly even with enormous towers that reach both sides of the city.
Anyway-- if anyone is interested, I'd love to hear more ideas about how things between these two settings could connect, and how things could be made to work as a fiction or gaming setting-- or why they wouldn't work, if you're opposed to the idea.
Is Sigil the biggest city in the multiverse? Is this actually stated anywhere in canon? I think Sigil may be, by some measures, the most important city in the multiverse, but I certainly don't think it's necessarily the biggest.
Consider, for example, the City of Man, described in Faces of Evil: The Fiends. This city is described as being more than eighty-one miles across. Area-wise, at least, that's considerably bigger than Sigil is usually said to be; unless it's very sparsely populated, it's likely bigger in population as well. (For comparisons of area: A circle eighty-one miles across has an area of somewhat more than five thousand square miles. A ring five miles wide and twenty miles in circumference (Sigil's dimensions according to Chapter 7 of the PSCS) has an area of only a little over three hundred square miles.) Of course, the City of Man isn't nearly the planar center of travel and commerce that Sigil is, and has much less importance on the multiversal scale, but in sheer size it's got the Cage beat by a fair margin.
Maybe there's somewhere I'm forgetting where it is specified that Sigil is supposed to be the biggest city in the multiverse (though if so apparently the writers of Faces of Evil missed that), but otherwise I think you're making an assumption here that isn't backed up by canon. Sigil has more portals than anywhere else in the multiverse (the City of Glass coming in a not very close but uncontested second). It may have more visitors from other planes than anywhere else. Sigil is certainly superlative in many ways. I just don't think plain size is one of them.