I would either write

Malvedar's Docent on Dragons, a dedicated Dragons sourcebook, or

D'Avenir's Travel Log, another Ravenloft sourcebook this time dedicated entirely to showing of a bunch new Domains, because I'm reading Van Ricthen's guide and I am not satisfied with 16 domains. Also more monsters.

Meepo’s Guide to Traps and Dungeons. Or Maps and Traps, for a punchier title.

Dungeon-building isn’t really covered well in the DMG, so more guidance on how to map, pace, and balance a great dungeon would be awesome! Plus, traps are also really underrated, and a Grimtooth-ish section about making fun and cunning traps, with loads of ideas from simple to complex, would be amazing. Flesh it out with a few monsters, magic items, and a Bilbo-esque “Treasure-Hunter” subclass that puts Rogues back in their dungeoneer role, and you’d have my ideal D&D book. It’s all stuff I really want to have more emphasis in 5e, but I need more guidance on incorporating it into my games.

I have to agree with the OP's verdict of more monsters. Volo's Guide II would be cool.

Also maybe a Dragonlance worldbook, Tolkien-style epic fantasy is still sorely underrepresented in 5e material thus far.

+1 for the guide to Maps & Traps. I am starting to design a few dungeons (all my adventures have been over land thus far) and a guide on pacing and ideas for filling up a dungeon, and making it grow organically rather than "this is a room with baddies in it, this is a room with a trap in it" would be very helpful.

"Advanced Monsters" would be a cool one. Cut some of the fluff and include Variants, Tactics, Personalities, and motivations for the existing monsters so that DM's can make them a bit more immersive than "these monsters were waiting in this room for you to arrive for a fight".

I've always favored, though am pretty sure I'll never see it outside of third party, in fact there's at least one 3rd party forthcoming book that does some of this work, some sort of world building tools that helped DMs design structures and encounter settings.  I've talked before about the AD&D Dungeoneers Survival Guide which spent a lot of time coaching DMs how to design their own under dark using a mix of actual natural science on cave formation coupled with the possibilities D&D magic and under dark denizens allow.  The Wilderness Survival Guide was a similar endeavor to coach DMs on designing realistic wilderness systems that would open a DMs eyes to the need for aquifers if you're having a forest, where and why you'd get snow peaked mountains, how a jungle would open up into a desert etc.  What permafrost is.  That sort of stuff.  And lastly a book that would basically discuss architecture and building principles for village, city, and fortification construction.  I don't think these sorts of niches fit into what 5e identifies as its sweet spot in terms of what would make a good book.

So that said, I would want less Tasha/Xanathar's "everything" type books and more Van Richten's focused type works.  The latter just seems to be a better book, and reading it I feel I'm reading something I can pick and choose from while also have the sense I'm reading a coherent work.  Tasha's and Xanthar's left me feeling I was reading a sort of miscellany laundry list type thing (with a really lazy effort to pull it all together with flavor text sidebars).  So given that, I sense some sort of Feywild type book (with lore, players options, monsters and maybe with how to run Alice and Wonderland type nonsense - there's actually math to it- as well as playing in an environment of overabundant emotions usually outside the spectrum of human exhibtion) or some sort of Dragon book like Dheubpurcwenpyl's Draconic Ditherings.  Actually I want to keep that name for myself.

There are really only two books I'd like to see at this point. A dragon sourcebook, and a feywild setting guide.

A new series of books: A wanderer’s guide to the planes. It would include books on: Feywild, Shadowfell, elemental planes, pocket dimensions and various outer planes.

 They would discuss methods of planar travel and what you might see on the other planes.

Quote from UndauntedDM >>

A new series of books: A wanderer’s guide to the planes. It would include books on: Feywild, Shadowfell, elemental planes, pocket dimensions and various outer planes.

 They would discuss methods of planar travel and what you might see on the other planes.

Man, that would be awesome! Given all the planar-related UA material lately (particularly the Feywild stuff) I kind of suspect something like this may already be in the works.

Quote from GrizwoldTheGoblin >>

Quote from UndauntedDM >>

A new series of books: A wanderer’s guide to the planes. It would include books on: Feywild, Shadowfell, elemental planes, pocket dimensions and various outer planes.

 They would discuss methods of planar travel and what you might see on the other planes.

 

Man, that would be awesome! Given all the planar-related UA material lately (particularly the Feywild stuff) I kind of suspect something like this may already be in the works.

 come to think of it,they may be bringing back plane scape

Definitely either Planescape or a Manual of the Planes. A lot of the planar material has been pretty sparce except for the Hells and sort of the Abyss. It would be a chance to bring back things like the guardinals, demodands, ect...

So my thoughts on "bringing back" Planescape.  I think if that happened, it would be in the shape of a book like the new Van Richten's Guide.  I think that would be up my alley in that it would be more focused than a Tasha's and service a particular type of adventurer, particularly planar hopping.  Here's the caveat.  Would we all be comfortable with each plane getting the page count given to domains of dread in Van Richten's?  Actually, there are less planes than domains of Dread presented, I think, so maybe they would be a bit more fleshed out.  

Actually I'm liking the format parallel.  A couple of planar hopping subclasses, maybe a Plane hopper or Berk background, write ups on the planes, a bestiary that fleshes out planar inhabitants.  Maybe something like Dark Gifts that pertain to character features that can be "picked up" or awarded in planar travel.  Running planescape campaigns could get into the ins and outs of planar travel, maybe actually reconcile the tension between the alignment systems foundation to the structure of the multiverse and the departure of explicit alignments being seen in recent books "Running Planescape Stakes Games" or the like.  It's like it almost fits a template VRGtR built out.

That said in an argument about Drow lore I just brain riffed on the idea of Nu Drow showing up in the FR in spell jammer ships, turns out they've always been around, but rather than around Faerun they've been tooling around the cosmos Captain Harlock style with old school sharp widows peaks and power staches set to maximum.  I would like a book that would help me ground that idea.

I'm doing my books in categories of Want to See (each category is a subset of the previous category).

There's nine books I think could happen (CH), six books I would like to see (LTS), three books I would adore seeing (AS), and a single book that would MAKE ME SO SO HAPPY (YAY)!

CH: A new adventure in Neverwinter; A new MTG setting book (Zendikar, Eldraine, or Kaldheim); A Dragonlance setting book;

CH, LTS: A new Mordenkainen-esque tome of monsters; A far-realm inspired campaign book; An Underdark book;

CH, LTS, AS: A Critical Role Campaign / Setting book (if setting, make it the one used in campaign 1 / 3); An Aerois setting book; 

CH, LTS, AS, YAY: A Far-Realm inspired Source book.

This is among others.