Spider Moon + Classic Spelljammer = Extra Fun
The original Spelljammer setting was chock full of interesting and bizarre ideas, from eel-headed spiders to blue-skinned giant merchants to walrus-tusked marines. While most of those classic concepts don't appear in the new Shadow of the Spider Moon D20 mini-game, there's no reason why you can't incorporate them into your campaign.
Below you'll find a selection of some of the best (or at least most infamous) elements that came out of the original Spelljammer setting, along with some tips on how to work them into your Spider Moon game.
Arcane, The: As noted elsewhere, these tall merchants almost made it into the Spider Moon game. (You can even find statistics for them, under the name "mercane," in Manual of the Planes and the Epic Level Handbook.) I envisioned them much as described in the original Spelljammer boxed set, as mysterious travelers in space coming and going as they pleased. It's up to you whether they remain the only source of spelljamming helms. They could well be the source of the ancient ruins on Ashen.
Beholders: None of the planets in the system detailed in Shadow of the Spider Moon are noted as having any population of beholders, though they certainly may lurk in dark places on any of the worlds. If you want to include the idea of beholders as a powerful spacefaring race, you should decide whether their empire is native to one of the listed worlds or if, like the illithids, they come from a distant system. A significant beholder population on Moradin's Forge might provide unusual allies for dwarven freedom fighters.
Crystal Spheres/Phlogiston: This is an easy one. If you have the original Spelljammer rules, and you want to introduce intersystem travel, simply use the rules you already have. Just because no one in this system knows about (or talks about) the existence of a "shell" around the system doesn't mean it can't exist.
Dwarven Citadels: While the remaining dwarves of Moradin's Forge aren't a true spacefaring race, it's easy to conceive that at some point in the past, the dwarves might well have ventured into the cold depths of space. Any remaining dwarven citadels are likely floating hulks, but these could be inhabited by anything from undead to formians to the descendants of a lost dwarf colony.
Giff: The militaristic giff could easily replace dwarves in your campaign, putting them in the role of a proud people who have watched their way of life collapse around them. They're also great fun with guns.
Neogi: While I hardly think that the system needs yet another horribly evil race of spacefaring sentients, there's no denying the horrible attraction of the slave-trading neogi. Depending on your tastes, you could place them as denizens of the deepest caverns of the Spider Moon--only now making their first encounters with the drow--or as yet another visiting race to the system, seeking a new source of slaves. Neogi probably make good sorcerers.
Pirates of Gith: If githyanki are cool, and swashbuckling space action is good, then swashbuckling space githyanki must be about the coolest thing around. Adding pirates of Gith to your Spider Moon game takes no work at all: just adapt the existing githyanki, as described in the Psionics Handbook or Manual of the Planes, to a spacefaring existence and you're ready to roll.
Scro: These tough orcs might be escapees from illithid breeding programs or simply the result of "survival of the fittest" in the dark tunnels of Moradin's Forge.
Giant Space Hamsters: In the depths of space, anything is possible. Originally bred as beasts of burden on the gnome homeworld, these bizarre creatures seem to thrive in the nomadic lifestyle of the gnomes. They also have a good nose for scavenging spare parts.