Oh, sure, there was the Astromundi Cluster - but that was only a single crystal sphere instead of a network of them, and what's worse it trapped visitors inside. And there were also supplements for the crystal spheres of Faerun (Forgotten Realms), Krynn (Dragonlance), and Oerth (Greyhawk) - but those always felt a poor fit, emphasizing Wildspace as a place to get from Setting A to Setting B instead of a setting in its own right and causing all sorts of problematic questions on why spaceships haven't changed those worlds in any noteworthy way.
So if I were to redo Spelljammer, I'd build an actual setting consisting of multiple star systems from the ground up. But more on that in a moment - if I were to redo Spelljammer, and I actually wanted to publish it, I would also have to alter many of the basic principles for copyright reasons. No beholders, no mind flayers, and no direct copying of ships and other prominent elements - instead I would only use the elements already published for Pathfinder or the OGL license.. Instead, I'd start with the basic premise:
"Fantasy heroes traveling through space in sailing ships."and see what I can build from there.
Let's start with how our sailing ships are moving through space. In Spelljammer, this was usually done via "spelljamming helms" - which required one of the spellcasters to give up all their spellcasting abilities for the day just so that they could move the ship. Bo-ring! How about moving them via actual sails which are enchanted to catch the solar wind - the ether emitted by the stars? (Implying that this "solar wind" travels fast enough to reach other star systems in a reasonable amount of time, but since this is fantasy we are not bound by real world physics...) Since the star ships require sunlight, this also implies that the easiest time to get off-planet is at dawn on the open ocean (since then the solar wind can hit the ship from directly behind), and the easiest time to land is in the late afternoon, approaching dusk, as the solar wind will then be able to slow down the speed of the ship (but woe to the ship which is caught in the planetary shadow while it attempts to land!) Needless to say, during night any star ships which have landed on a planet are stuck there.
As the ether is emitted by stars, planets can cause "ether wakes" (stealing from Space: 1889 here) which make travel difficult - and not surprisingly, space pirates will frequently lurk there. In deep space, ships generally have too great speed differences to intercept them directly. Thus, to intercept another ship, you have to (a) be faster and (b) explode some "ether bombs" in its path which cause localized ether disruption which force the other ship to lose some of its speed.
Between star systems there are vast currents of ether which ships can use to travel between them (using cross-sailing if they have to go in the wrong direction). Of course, given that the stars are the sources of ether, there also must be sinks. These are the "dark stars" which absorb the ether and slowly grow larger over the eons. As their mass grows, they also accumulate stellar detritus up to and including rogue planetoids and planets. Needless to say, only the most desperate humans are willing to travel to these locations (which is why they make swell pirate bases and the like) - but there are all sorts of nasty life forms which thrive there.
Which brings us to the actual setting. Spelljammer got a lot of its imagery from the Age of Sail - so let's model the worlds of the setting on the actual Age of Sail. In general, the following regions exist on the star maps:
- Core Worlds: These star systems are the seats of a number of interstellar empires, some of which are clear analogues to the real world empires of the Age of Sail while others fill out some of the more common fantasy tropes (like the home worlds of the Elven Space Armada and so forth). Presumably, there used to be an older "Precursor" empire which scattered the humans across different world (whether or not those precursors were human is another question I will need to examine later). In general, these worlds are highly developed and orderly, which is not the same as "peaceful", let alone "free of intrigue".
- Colonial Space: The various stellar empires compete for colonial worlds and their treasures, and thus there is a wider ring of such planets and star systems surrounding the Core Worlds. There will be major settlements which are fairly civilized, but the whole area is still a fairly chaotic frontier. Needless to say, this is where pirates and privateers will be the most active, as the trade routes are the most profitable here.
- Wildspace: Here Be Space Dragons. The distant star systems beyond the reach of the empires of the Core Worlds. Explorers may go there, but only heavily armed. Anything might be out there, including entire civilizations of the more disturbing creatures.