To perhaps reinforce the horror element, that cover art is pretty horrific, and I've seen some online speculation that the owl-creature on the cover is a hill giant vampire named Morg that was created by Paul F. Culotta in Dragon 236 (he could turn into a giant owl). I looked up the article he...
I'd imagine any 5e planar book would be an examination of the Inner Planes, Ethereal/Astral Planes, and Outer Planes in the Great Wheel configuration (with nods to demiplanes, the Far Realm, and alternate cosmologies like the World Axis and Mystara's) heavily influenced by the Planescape setting...
But they did a planar AP last year. I didn't think they'd do two in a row (though maybe you end up in the Paraelemental Plane of Ice or Pandemonium in this one).
What are you counting as APs? If you don't count the anthology adventures then there's been one storyline per year since 2017 (2017 Tomb of Annihilation, 2018 were the Waterdeep adventures, and 2019 was Descent into Avernus).
And that's the right attitude to have IMO. I think if there is an artificer reprint, it will include one of the subclasses from E:RLW - I'm just going to assume the alchemist.
It doesn't take much prognostication to see that WotC is planning a book with an emphasis on player options similar to XGtE - I would say it's probably the 2020 Q4 release (I also don't think it will include psionics). That said, I wouldn't expect that it wouldn't include as many subclasses as...
Given the proliferation of UAs over the past few months, we'll be seeing a Xanathar's type book in the near future. I would venture to say it's likely the 2020 Q4 release.