I'm going to maybe commit heresy here, but going out on a limb, I think I could say "Nobody gives a rat's arse about 80% of the Planes", and I would apply that especially to the vaaaaaaaaaaaaaaaaaaaaaaast number of people who have come new to or back to D&D with 5th edition.I said Manual of the Planes because I feel all of the planes need more attention in 5E, but the book I would get the most mileage out of would be a Spelljammer book, mostly because I could use stats and mechanics for the ships even though I use them differently.
This is why I think a specific Manual of the Planes, which carefully lays out the Great Wheel cosmology (or worse, tries to lay out multiple cosmologies) and dedicates roughly equal space to each plane and so on would basically be completely useless to the very vast majority of DMs and groups. First off, you'd just be re-iterating stuff in the MotPs of previous editions. Second off, most it is incredibly boring and actually actively antithetical to adventuring. Even Planescape struggled with this a bit, and only the deal where stuff shifted planes depending on beliefs, together with inventing the Blood War, really gave any reason to engage with most other planes.
It's like, I think even a lot of people who might say they want that, wouldn't actually get much or in some cases even any usage out of it. They might enjoy reading it, or looking at it on their shelf, but I don't think would be, like, a thing people would find useful.
Whereas a Planesjammer approach, which maybe didn't even completely clarify the Great Wheel, or dare I say it, maybe didn't use the Great Wheel (but still had a reason for Sigil to be at the center and City of Doors and so on), and didn't like, spend aeons trying to explain how much fire damage per round you take on the elemental plane of fire, or why nothing at all exciting can ever happen on any Good or Law-aligned plane, nosiree, but rather focused on Sigil and adventures based out of there, to highly exotic and wild planar locations, rather than boring as hell ones, and really focused on the game-able, engaging, exciting stuff, not the dreary recitation of facts or sad attempts to make dull stuff exciting (yeah I'm looking at you other MotPs!). Have spelljammer-type ships traversing the astral or other planes would be pretty effin' awesome, and because it wasn't exactly spelljammer you could throw out what didn't work from that, and bring back in what did (which would necessarily include the Giff, Neogi, etc.). Someone earlier said make it it's own setting, not just an expression of the Great Wheel and I think that's right.
Maybe you could title it "Planesjammer" (because it would be fine to call the ships that) and have the tagline: "Re-invent the Great Wheel!".