Perhaps I'm in the minority, but what I don't like about SpellJammer are the pseudo-antique physics. I'd much rather have a "D&D in Space" supplement that used up-to-date physical assumptions. I dig the space-faring sailing vessel idea, but it should have to be insulated against vacuum and radiation. You should have to face 'mundane' hazards like black holes and pulsars as well as the fantastical stuff.
I'd keep things in SJ mostly like they were, but I'd have it be D&D in realistic outer space. So part of it would be solving the physical challenges of space exploration with magic and ancient technology. Plus there'd be swashbuckling, wizard's duels, gun hippos and all the other stuff. A monolith would show up at some point.