Selljammer

what is 4x?

Explore, exploit, something something. Video games like the Civilization series. Or Masters of Orion.

A game based around helping to establish colonies in new, strange, and often hostile spheres where magic is not necessarily going to function as it did back home, among many other issues.. then defending it against pirates, hostile locals, or even imperialist folks there to seize it once it's up and running.

Rather, a game based on all that would be a fun one to run or play. Priests have to establish a local bastion of their faith to get their full powers, mages have to study some new ways to cast perhaps, or other things of that nature. Exploration, discovery, experimentation, and overcoming conflict would make great key themes for a game like that.

can spelljammers sail through space in the conterminous planes, like the etheral and shadow, in 2e?

Also, if the phlogistin is between the spheres in the material plane, what is between the etheral spheres in the etheral plane?

I would imagine they could sail it, yes. The deep ethereal is what's between the border ethereal and other planes, including prime worlds, the inner planes, and demiplanes that lie deep within the deep ethereal.

Ethereal is a-ok for spelljammer use. You can go into the ethereal while in the flow. It's just normal ethereal space, though. And without the flow to travel between spheres? It's just a big, empty nothing.

I assume you mean Ravenloft by shadow? The demiplane of dread is in the Ethereal. So sure: spelljammers should work just fine in it. Would it ever let one in? Eh. Prolly not. Although it could make for a fun demiplane: a giant ships' graveyard of people taken into it out of the flow. Ships all crashed together, spelljamming impossible for no aparent reason. Just a black nothingness with a core of immobile ships all stuck together. An urban space in an endless void where gravity becomes problematic on occasion.

Pretty sure there is no Ethereal coterminous to the plogiston. That's why extraplanar spells that rely on the Ethereal still don't work.

You literally jam spells into the ship to make it go

I like this map more.