I want to give Sharn the Atlantis treatment

[MoogleEmpMog]

A spelljamming ship requires a spelljamming helm, which is essentially a thronelike minor artifact that transforms spellcasting into spelljamming. These artifacts are only produced by a race of enigmatic blue-skinned giants called the arcane (the Epic Level Handbook reintroduces the race as the "mer"cane if you need 3e stats), but of course in the scenario I outlined Merrix d'Cannith's creation forge can produce them.

Other than that, the ship can take virtually any configuration, whether it's capable of traveling on water or not. Most really good spelljamming ships, like elven man-o-wars, are not seaworthy.

Some notable spelljamming factions, and their ships:

The Elves
Elves in Spelljammer are basically stereotypical Victorian British imperialists - high-minded, sure, but arrogant as hell and quite willing to let everyone else know about their superiority. They are masters of plant-based biomagitechnology and actually grow their ships, which are gigantic butterfly-like vessels of living plant matter. The elves have wide-spread influence across wildspace.

The Goblinoids
The goblins were the most powerful force in the Spelljammer world about a century ago, when they fought a massive war against the elves and lost - the Unhuman War. The goblinoid races are still powerful, especially with the rise of the scro, a race of intelligent, organized hybrid orcs. Their most powerful weapons are also living, a kind of giant ooze-like kaiju called a witchlight marauder.

The Ilithids
Unlike most settings, Spelljammer leaves room for ilithids to interact with other races on a peaceful, though not friendly, basis. Spelljamming mind flayers employ ships that resemble nautiluses, although they are actually constructed from mundane wood. The mind flayers trade and make treaties with the other races - quite a shock to anyone hailing from Eberron!

 

[MoogleEmpMog]

Air Envelopes and Gravity Wells
Spelljammer doesn't employ real-world physics. The original boxed set brilliantly described the setting - it is a world with consistent and 'real' physical laws, but they are the laws of Eliminster and Mordenkainen, not Einstein and Newton.
One consequence is the way air envelopes and gravity wells are handled in Spelljammer. Every ship has its own air envelope, so characters can walk around the open deck of a ship and breath deep of the (usually stale) air. Battles between spelljamming ships often involve attacks on that air envelope, for example with cloud kill spells.
Another consequence is that every ship has a gravity well (full Earth/Toril/Eberron Gs) running through its median point. You can walk on the bottom or the top of a ship and feel like you have normal gravity (some ships take advantage of this), but if you're dead center you'll feel like the gravity is unnatural.

Crystal Spheres and Wildspace
Every existing gameworld is contained within a crystal sphere in Spelljammer, in the area referred to as wildspace. A crystal sphere includes all the constellations and other stars seen from the planet, many of which have divine associations. Every crystal sphere has a name, usually referring to the campaign setting it contains or the primary planet thereof - Realmspace, Krynspace and Grayspace being the most prominent, with Eberrspace becoming a significant player if you want to add it in. Many other spheres exist as well, either unaffiliated with existing campaign worlds or serving to house the d20 worlds of your choice.
The crystal spheres themselves are ineffable and indestructible. However, spelljamming helms give their users the ability to open a temporary hole in a sphere, large enough for the ship to pass through into the phlogiston.

The Phlogiston
This might be the "weird space" you wanted Sharn to be in - and it would have to pass through, if you want it to end up in Realmspace!
The "Flow," as spelljamming regulars call this strange environment, is an unusual environment utterly devoid of planets and native life, aside from some truly alien entities. It is a fathomless miasma of brightly colored luminescent gas, all of which is intensely flammable, making travel extremely dangerous.
Creatures cast adrift in the phlogiston do not die, but instead enter a form of suspended animation, sometimes for centuries or even millennia. An object the size of Sharn is likely to pick up many such castaways.
Spelljamming ships pack as much air, food and water as they can cram into their holds and break across the Flow, seeking to travel from one crystal sphere to another. Because most ships cannot pack in enough supplies for longer journies, they tend to 'jump' from one sphere to the next, often taking months to get from one planet to another in a distant crystal sphere.

 

[MoogleEmpMog]

Sharn In The Phlogiston

While in the phlogiston, Sharn is at significant risk. The three primary dangers to the city are: planar cutoff, erratic magic and native entities.

Planar Cutoff

While in Eberrspace, Sharn faces no danger to its air envelope - it's tied directly to Eberron's plane of air. Once it enters another crystal sphere, such as Realmspace, its connection should probably resume, although it might behave strangely.

But while in the Flow, Sharn has no connection to the outer planes. The phlogiston doesn't play nice with extraplanar travel, and gates to and from it are unstable at best. During its long and undirected odysse through this alien environment, Sharn will be faced with an air supply that dwindles with every passing day. By the time it reaches Realmspace, the city will either be a polluted mess unless its scions can find a way to cycle or purify the air.

At the same time, any extraplanar entities that may reside in Sharn will be cut off from their home planes.

Some of those entities may be benevolent, celestial creatures native to Syrania. They might be able to help with the air situation.

Others are probably Daelkyr or their minions. Though not likely to be helpful, they may be able to help themselves - ilithid, neogi and beholder vessels ply the flow, and all three races likely have their representatives amongst the children of Xoriat.

Finally, any Inspired in Sharn will probably be in quite a fix! Their Quori possessors will either have part of their essence trapped in the city (perhaps allowing them to manifest physically for the first time in countless millennia!) or be thrust back into Dal Quor, leaving the Inspired unpossessed and free to do as they wish. If being in the phlogiston allows the Quori to manifest, this may have been their plan all along. Especially if they stay manifested after Sharn leaves the flow...

Erratic Magic

Some spells don't function properly in the phlogiston - mostly planar travel or planar communication spells, but some general utility spells as well. Fire spells are incredibly dangerous, because any flame can cause a massive explosion (equivalent to a CL 20 fireball).

Most spelljamming vessels don't mind this much. But Sharn, an entire city, an Eberronian city, is full of constant magical effects. Some of these may behave erratically, cease functioning, or even malfunction in some devastating manner.

Native Entities

The natives of the phlogiston are creatures so strange and alien that they can barely relate to ordinary life forms. Often, what relations they do have are hostile. Almost any strange creature can be incorporated as a Flow monster, but oozes of various types are particularly appropriate.

While encounters with the natives aren't likely to destroy the city, they certainly won't make the Sharn citizens' stay any more pleasant. Perhaps worse, their activities could cover other, more fathomably malicious actions on the part of mundane villains, trapped extraplanar beings, or even spelljamming raiders.

 

[Plane Sailing]

Lots of spelljammer ideas here, and I can't speak to that as it's outside my knowledge.

But just in case it is useful, I'd like to add an additional possibility.

What if the manifest zone to Syrania was suddenly strengthened, and Sharn was actually pulled into Syrania? What if the angels, for some reason of their own, are behind this? The "I, Robot" film used the premise of

Spoiler

a being constructed with ultimate good in mind (three laws of robotics) being able to contemplate and execute nasty plans in order to reach the greater good

- perhaps a cohort of angels decided that the only way to stop the meaningless deaths and chaos in Sharn was to bring it under their direct supervison and restriction of freedoms...

How noir-ish would it be for the bad guys to be good guys who were over-zealous, over-powerful and over-here?

Cheers

 

[Dr. Strangemonkey]

All useful stuff, thank-you.

Now, how does spelljamming actually propel people? In jumps or smooth lines?

Are there means to do jumps?

IE could you do a SDF style fold followed by lots of travelling by more conventional means?

Do the crystal fears then essentially act as focuses where by non-material planes can interact more readily with the prime and thus bring about ordering the phlogiston?

 

[reanjr]

Eberron's cosmology might be a good place to look. Maybe a plane of existence bonds with Sharn and when it moves away from Eberron, Sharn goes with it. As the plane meanders and comes in conjunction with other planes, Sharn is transported with it (and might even be left on one of these planes).

 

[MoogleEmpMog]

All useful stuff, thank-you.

Now, how does spelljamming actually propel people? In jumps or smooth lines?

Are there means to do jumps?

IE could you do a SDF style fold followed by lots of travelling by more conventional means?

Do the crystal fears then essentially act as focuses where by non-material planes can interact more readily with the prime and thus bring about ordering the phlogiston?

Ordinary spelljamming does not work in jumps but in smooth lines; in the canon Spelljammer material, the only way to do jumps is to have a powerful spellcaster teleport you, and that only works within a crystal sphere.

However, in any Eberron-Spelljammer fusion, the presence of a vast source of energy (the Siberys Ring) in orbit around Eberron could cause all manner of strange effects, possibly including an initial 'jump.' Whatever malfunction (if, indeed, it is a malfunction) initially sends the entire city careening into space could also cause this. You could, in this way, get the city directly into the phlogiston (bypassing Eberspace) where it could endure the dangers and tribulations there until drifting to Realmspace.

The crystal spheres seem to act more as a shield against the volatile phlogiston than anything. However, some spheres, like Krynspace, appear to have an internal cosmology unto themselves, even on the planar level; this could be as simple as the manifest planes following the same model as the mortal perceptions of the sphere, if you go with Planescape's explanations of planar form. Eberspace should almost certainly have a similar effect, although I personally would set most of Eberron's planes (not Xoriat and Dal Quor) on the planet's actual moons, perhaps with the Siberys Ring acting as some sort of transfer mechanism.

 

[Dr. Strangemonkey]

Cool.

Now, how do the gods work around this?

There are gods that move between settings, do they just bypass the issue by using connections between outer planes or is there something else going on?

 

[MoogleEmpMog]

2e limited gods even more than it limited class levels by race - witness the godless wasteland of Dark Sun's Athas, Planescape's Lady of Pain cheese, the dreadful fates of FR and DL divinities, and Vecna's bogus journey!

Spelljammer got in on the act, restricting gods to their own crystal spheres except where they were obviously worshipped in multiple spheres. Even the latter group couldn't project their power from one sphere to another, though; the crystal spheres were structures beyond even godly power.

But Spelljammer, unlike much 2e material, actually explained why its gods were thus constrained: it operated under the worship = power model, most gods were venerated in large numbers only in their home spheres, and those who did cross over, like Corellon and Gruumsh, did so because their worshippers had a long and storied history of spelljamming.

A god needed a strong base of worshippers (such as the spacefaring elven armadas and colonists) to spread his influence from one sphere to another. To some extent, the Unhuman War appears to have involved a godly power grab as well as a mortal one.

Now, if Eberron is interacting with the other worlds, an important issue must be addressed - what's up with Eberron's gods? They don't manifest, they don't walk, they don't talk. A native of Krynn, Oerth or Toril would probably assume such gods don't exist at all. But none of their gods have moved in to mine the massive worshipper base Eberron represents.

I would personally bring the Siberys Ring back into play here. Eberron's antediluvean uberdragons were beings well beyond the power of a typical D&D god. "Ordinary" gods can't manifest or directly involve themselves on Eberron because of the massive magical energy field surrounding the planet (and permeating it in the form of Eberron and Khyber).

 

[Dr. Strangemonkey]

In your opinion then, what would be the effect if Sharn were to shoot through said field?

Consider two results:

1.) that Sharn carries with it a similar but smaller field?
2.) that it also tears a whole in the field that surrounds Eberron?