Preview images of the upcoming Spelljammer boxed set have been appearing on sites across the web, so I've gathered some of them together here. These also include the hobby store 'special edition' with Hydro74 art.
I had this idea that would potentially preserve the original Phlogiston, while still making Astral travel the default:
As you approach the edge of a system, what you encounter depends on how fast you are going. If moving at spelljamming speeds, you pass through a massive portal to the Astral Plane (essentially, the portal key is moving at spelljamming speeds).
If you are moving more slowly, or stop before passing through the portal, you encounter a hard crystal sphere. This can be bypassed by magic or through naturally occurring openings, and after that you're in the Flow.
Travel via the Phlogiston is actually faster than Astral travel, but has several drawbacks. All the flammability issues of the old Flow are in place, so it's dangerous. There's also little to nothing out there, so no chance to resupply or otherwise receive aid. Finally, those creatures which do exist in the deep between spheres are unknown and perhaps unknowable, think Great Old One type stuff. Most folks far prefer the Astral for a safer, if slower, alternative.
Not a bad price compared to Amazon. I pre-ordered it a while back and as of now its $45.66 USD. There's one game store a few blocks away from my house but the couple of times I went there they were snobby and seemed like they couldn't be bothered. My neighbor said the same thing when we talked about possibly getting a game together or joining one there. Seems odd to me as you'd think they'd do all they could to get and keep new customers. It's just easier for me to order online.
How so? The Astral map we've seen only shows the Material and Astral. You could presumably use a color pool, but those are rare and may be too small. And the ship in thr BG3 intro is apparently using plane shift spells, and not Spelljamming in the way we're being told is how it will be in the new product.
Of course, I will have to read the actual rules when they came out, but if that turns out to be case, I'll consider having the Astral Sea as something separate from the actual Astral Plane and to enter the latter you'll need extra magic or find a portal.