Spelljammer …Wait till you see Waterdeep: Dungeon of the Mad Mage (Spelljammer?)

Is that Boo the space-hamster I see?

 

[the Jester]

It's probably worth mentioning that in the last 'Happy Fun Hour' someone asked Mearls about whether the subtle Spelljammer references meant something. And he responded by saying something like (not exact quote!) "Subtle? I don't think the references are subtle at all!". Now, he did NOT then say it meant there was a product. But clearly he (and Stewart and Perkins) are really enjoying dropping all the Spelljammer references.

They clearly have some plan in mind.

Well, both 3e and 4e had quite a few references to SJ, but never did an actual product for it. For instance, the 3e book Lords of Madness included a crashed neogi mindspider and the 4e included spelljamming ships as a method of planar travel. So although I'd welcome a 5e Spelljammer expansion, I wouldn't count on seeing one... sadly.