Spelljammer was never my bag. Not all great tastes taste great together. There's a certain type of genre mixing that just doesn't pop off for me, and getting on your magic spaceboat and flying from Dragonlance to Dark Sun to Greyhawk to Forgotten Realms never sat well with me. I like these worlds better without visitors from another planet arriving on their magic spaceboat. I'm not much more enthusiastic about cross-pollinating these worlds through planeswalking, either.
I've lightened up a bit over the years. Spelljammer as a concept seems fine, but it kinda needs to be its own thing. Everything I've heard about the original Spelljammer has been positive, and that it's a fun gonzo adventure. I'm willing to take those reviews at face value.
I wanted to enjoy Planescape. It definitely set the cosmology in D&D. There were a few things about it that didn't jive with me. I didn't really like the factions and I wasn't all that interested in Sigil. Looking through the boxed set, I'm seeing the cracks. Some quick head math has this boxed set with about the same page count as the 3E Manual of the Planes. Over half of the boxed set is Sigil and the factions (so basically, Sigil). It doesn't really talk all that much about the other planes. If you don't have the other boxed sets (which I didn't back then) that's what Planescape is - Sigil. The other planes, which should be the interesting part of the setting, feel like a footnote. I think if the Planescape boxed set had contained the material that's in Manual of the Planes, I'd have liked it a lot more. All the infinite planes, and we're talking about this one city in the Outlands. Which, because it sits in the center of the plane that sits in the center of all the other outer planes, is the most important place in the multiverse. It makes Planescape feel small.
If I sat down and read through this again, I would probably like the factions more now. Having read Manual of the Planes definitely helps with that, and I can see how somebody who had the 1E version of that book might have been more interested. I now know things about those other planes that aren't presented to me in the original boxed set. I'm sure being in my 40s rather than high school makes a difference as well. I'm not a big fan of how all of the D&D game worlds are kind of shoehorned together to fit the Great Wheel. I prefer allowing each world to have its own cosmology that doesn't have to fit neatly with the others, but that's a tradeoff that probably has to be made in order to do Planescape as a campaign setting.
If WotC is going to do Planescape, I'd prefer for them not to try to do 2E Planescape. That's exactly what they're going to do, and it's going to be terrible, because WotC fundamentally doesn't understand what fans love about their favorite game worlds. This is entirely separate from any problems they have with the Seattle virus. WotC mangled the Forgotten Realms from 3E-4E-5E. What they're doing to Dragonlance also looks stupid. Trying to rewrite old Greyhawk modules to cram them into the Forgotten Realms is stupid. It's like they don't have anyone who knows how to write fluff, and they keep wanting to change fluff thinking it doesn't matter and nobody cares.
Planescape is built into WotC's flagship product. Release a full length book on Dominaria. That's the default game world where all your modules are set in. Your "Prime Material Plane". Then release Planeswalker's Guide to the Multiverse. Bam. 5E Planescape. Yeah, the old heads won't be happy that they didn't get their Planescape updated for 5E, but they all know they weren't going to get that, anyway. At least they won't be pissed off that their cool setting got butchered and mangled by incompetent buffoons.