I agree with your genre breakdown of Spelljammer and Planescape 99%!
But . . . blending Spelljammer "fantasy space" adventure with planar adventure is still a great idea. It just wouldn't really be Planescape. Planescape was just as much the weird planar locales as it was the weird planar factions and weird tone. We had planar adventure in D&D before Planescape, we can have it again without Planescape. And there is plenty of inspiration from fiction for the ship that travels between worlds . . . .
That said, I really do want WotC to do a true Planescape release for 5E at some point. Focused on Sigil, the factions, and all that weird fantasy goodness. I think I might go boot up "Torment" tonight, and revel in some weird fantasy nostalgia . . . .