Spelljammer can work well as a "transitive setting" taking you from one world to another. The original boxed set is based around connecting up Greyhawk, Dragonlance and Forgotten Realms.
The phlogiston connects the different settings, but the crystal spheres within the flow, give each gameworld its own "planetary system". What is great about Greatspace (if you will excuse the pun) is that the Hellenic theme means that you could connect it to other spheres or you could have a series of adventures flying a ship around the various parts of this one sphere.
Something I've been reading about recently, is that Dave Arneson took a map of Holland, spun it around and turned it into Blackmoor. I wonder if it would be possible to play with a map of Greece and turn it into the one of the worlds of Greatspace. Maybe other Greek things (like town plans) could be rebooted and used for other maps.
Given that Thesalys is the world rules the sphere I think it might be best to make its map a modified version of Greece.
I did a quick search and found this map of Greece and its neigbours. That doesn't look too easy to use as a world map, as countries like Turkey fill one edge, but look at this other map that only shows Greek lands. That seems a bit more believable as a world map because it is "surrounded" by what seems to be sea. Rotating the map 90 degrees clockwise could help make this less obviously a map of Greece.
There are several types of space dragons, but a conventional dragon can also fly into space. A "groundling" dragon could not fly far, but it might get from Thesalys to its moon (Myle).
There are a lot of asteroids in Greatspace's asteroid belt and plenty of different monsters in Greek mythology.