The Unearthed Arcana, obviously, should be a book with the same kind of material...not the same material. i.e. I don't expect/want/need to see a Barbarian variant, Cavalier and Acrobat...But a few new archetypes, for several classes....maybe [and it's a big maybe] a new full class. The new spells, the new magic items, the rulese variants. Yes absolutely.
The Fiend Folio should be the "second monster book". End of story. I clicked on FF but not MM2. Take the stuff you want from FF and from MM2, maybe a few newer things and turn it into /the/ second monster book...as someone else said, of the quality and structure of the existing MM. We don't need both. But a 5e FF would be great.
The Wilderness & Dungeoneers Survival Guides, I think, could /easily/ be combined into a single "Survivalist Guide" manual and do great. I want that, so checked off both of those, but don't actually want /two/ separate books for it. That's unnecessary.
I agree that secondary [tertiary, four-siary

, etc...) setting books are great, so Oriental Adventures gets a nod. Offering a battlemaster (or paladin? "Oath of Honor" anyone?)/samurai and shadow monk/ninja variant, plus clerical and wizard schools for your sukenjas, et al. Other monk variants, different magical traditions...maybe introduce those turtle and fox people and [hawk?] kenku/tengu as playable races. Also, I wouldn't mind seeing a bit more "pan-asian" than just "Feudal Japanese." Give me some mongolian riders, give me some chinese/kung fu martial "Fighting Styles" for warriors, give me, even/maybe Indian options, and of course different/new backgrounds, [limited] feats, a few spells (I think the elemental themed PotA spells cover a lot of ground already), etc...
Having just recently perused my 1e Manual of the Planes...to be honest...I don't want it. The little planar synopses in the DMG pretty much cover stuff. I don't need a whole book just to give me a plane by plane "This is what color your portal should be" or "XYZ deities live here"...If they did it...it should really be VERY different from the 1e MotP...I could see/endorse a planar "adventure guide" that's really useful...random environmental effect charts, monster/encounter tables, MM-style entries for things like the Sultan of the Efreet, archons/other angels/extraplanar beings that aren't in the MM, and that sort of thing...But I really think this is probably better suited by a series of articles/smaller supplements: "The Inner Planes", "The Higher Planes", "The Abyss", "The Hells", etc... And for the love of whatever powers you hold dear, LEAVE PLANESCAPE THE FRAG OUT OF IT! ...OR, just put out a Planescape Setting manual and forget a "Manual of the Planes".
And, just for nostalgic fun, I'd love to see a Deities & Demigods for 5e. Heck, that Sultan of the Efreet business could be included in there...uber-powerful-but-not-deified-beings [handy for warlock patrons? hmmm?]. I would go light on the real world, just hit the major 3. Through in a healthy entry for Greyhawk, FR, Krynn & Eberron. IF they could do it properly/get the rights this time, Tolkien, Lovecraft and Howard sections would be cool. A separate chapter on "religion", types of religions/belief systems/philosophies, religion structures/institutions...basically using/incorporating deities, religion, and mythic themes into your gaming. And, of course, tips, criteria and samples [CRing?] for creating your own [fantasy] pantheon that delve a bit further/more detailed than in the DMG.
[EDIT to add to the DDG, as following posters have noted and I completely forgot about: Non-human pantheons/deities! Dwarves, Elves, Halflings, Gnomes, of course. Dragonborn need a pantheon [?] or at least the Bahamut/Tiamat duality. Orcs, I suppose, though I don't see them needing more than 1, really. One for Goblinoids, One for Lizardmen...Koa-Toa, of course (can't have a DDG without lobster-headed unpronouncable chick

Ummmm dunno if you need more than that. This would, of course, need to include rules/options for "Shaman and/or Witchdoctor" NPCs...if not a PC usable class. Maybe a passing mention for demon-worshipping monsters: like Gnolls, Minotaurs, Lamia, etc... No need to put demonlord stat blocks in here too...though if they aren't anywhere else but the OotA adventure path, I suppose they do go here...unless there's a manual of the planes. /EDIT]