That sounds more like planescape than spelljammerA chapter on Sigil
An adventure within the Gith war
A class or two. I would also settle for subclasses.
A playable race or two.
Altered spells of the Astral Plane, in particular movement and teleport spells.
Rituals that involve the planes.
Transportation through the planes.
Monsters - lots of monsters. Maybe an Astral purple worm.