The thing about *blank*jammer, is that it's more exciting when you have places to go. In 2e, they absolutely would dedicate entire and separate product lines for Spelljammer and Planescape to detailing those destinations, but that's just not in the 5e model. They're not likely to have Ravenloft & the Feywild over here on their own planes, and Eberronspace and Faerunspace over there on a single prime material. I suspect that they are more likely to leverage what they have. And if that includes the Magic The Gathering settings, which are by definition different planes, Planejammer becomes more likely. That being said, I think that we are more likely to see something that evokes classic Spelljammer than just putting ships on the Astral Plane.