The aether is the substance that exists between planets and other bodies in outerspace. It is the medium through which light and very small amounts of gravity flow. Like a great ocean, aetherspace has endless eddies and currents. Most notably, currents run to and from stars to planet sized bodies that orbit those stars. Stars also produce a solar wind that blows in all directions, not just along the paths of the aetheral currents.
Any aethership takes advantage of these currents as much as possible. Sails are also in use for tactical speeds and maneuvering. The main propulsion is provided by a vestibule, so named because it is a bridge between the world and the space between where all aetherships go. (a sort-of hyperspace) It is a specially crafted room on all ships.
You might call it M20 Spelljammer. Please don't.
The thing is that no part of SpellJammer is or was Open Game Content. The d20 mini-game in Polyhedron 151 is great, but as stated on the first page, NOT open game content. 100% proprietary. Andy Collins' site has references to material left out of the Polyhedron article, but all of that is also NOT open game content.
The name Spelljammer itself is a registered trademark of Wizards of the Coast. This post will not challenge that trademark. Other items of the setting are also trademarked and proprietary. I say this because SpellJammer cannot be used to reference the idea of magical spacefaring adventure despite the similarity in concepts.
As such, let me begin by saying I want to extend the M20 ruleset (including standard races and classes) to an extra planetary setting. The working title of this setting will be Aetherflow, abbreviated AF. I will attempt to write this from scratch using other M20 materials developed by others as well as my own ideas.