Manual of the Planes: This book will be written pretty much just like it was in 3E. No changes here.
Oriental Adventures: Pretty much just like it was in 3E, too.
Epic-Level Handbook: Obviously the mechanics of the book will be much changed. And a greater emphasis will be made on coming up with suitably-epic adventures for the PCs (there will have to be TWO adventures, one for starting epic characters and another for epic + 3-5 levels). The NPC list was a fine idea, but a little less wanking would be nice.
Book of Vile Darkness: Also pretty much the same. However, there should be less of an emphasis on gross-out and more of an emphasis on grimdark. Less Drakengard, more Vampire.
Arms and Equipment Guide: Obviously, there will still be some magical items and mounts and combat equipment and all that to slake the thirst of powergamers. However, there will also be a lot of stuff in the book about just generic items in GENERAL. Stuff like ships, the price of blocks of salt, plenty of information on traps, and obscure-but-fun items to have in it like the spyglass and sextant.
Enemies and Allies: I made a thread stating the changes that would have to be made, but definitely have one of these books out.
Stronghold Builder's Guide: The iteration of this book for 5th Edition should be a lot like the 3E version. However, there needs to be more of an emphasis made on strongholds or buildings accessible to low-level heroes. There's no reason why a 4th level (out of 20) hero shouldn't have a nice wooden fortress armed with competent guards if they really care about such a thing.
Unearthed Arcana: This book obviously needs to come near the end of 5E's lifecycle. Basically this book is a compilation of houserules and playtest feedback accumulated throughout the edition.
Deities and Demigods: Less wanking to invincible supergods, more wrestling Amon to the ground and giving him a shot in the pills. Awww yeah boy.
Cityscape: Sort of like the 3E book, but completely different. You can read in this very thread the fail of this sourcebook and the suggestions for what it should be.
Maltheopia: This book will be the 'evil' counterpart to Cityscape. This book will have a bunch of stuff on living and surviving in evil cities (especially as a good character). There will be lots of suggestions for antagonists and organizations one might expect in Killfuck Soulshitterville. There will also be a lot of evil-city threats such as urban wendigos.
Wildlands: For our terrain books, writing compellingly about grasslands and wastelands is probably the hardest. So instead of wanking to the terrain, this is the book where we build and describe a fictional empire. In
this thread I talked about creating an evil barbarian empire. This book will be primarily highlighting whatever we come up with here and will take great pains to detail about how savage and exotic the culture is.
Silent Desert: Like Sandstorm, but a little less ass. Like Wildlands, there is only inherently so much you can do to make the Saharan desert interesting so this book will mostly focus on the cool shit you can find in deserts and the organizations rather than the desert itself. I know you can talk about other deserts like the American West but we're saving that for another book.
Frozen Frontier: Like Frostburn, but a little less ass. You can do significantly more with the terrain in this setting than the other two because you also have shit like forests and mountains and underground ice palaces to talk about.
Field of Blades: This book should focus on fantasy warfare. Remember that blurb at the back of Complete Warrior that deconstructs typical European medieval-fighting with D&D mechanics? Like this; the book should give the whole Bronze/Iron Age wars the old D&D once-over. I'm talking artwork of goblins-on-worgs shooting down a fleeing troupe of halflings who had their line busted while gnomes prepare an alchemy ambush. In the very likely event that we don't get a good mass-combat minigame working in the basic rulebooks, it should go in here.
Forever Forest: Our (duh) forest book. This should be the easiest book to write because when you think of standard fantasy terrain, you think of fucking forests. Also you can have some swamps all up in here, too.
New Horizons: In this book we introduce some
Steampunky Gaslamp Fantasy elements to the game. It's a new era and shit, airships are being built, people experiment with democracy seriously, and horror of horrors we actually have GUNS and shit! Oh, the humanity! But basically this book is supposed to be the setting which highlights a fictional transition from the Late Medieval Europe to Late Reinassance Europe. The theme of the book is almost completely exterminating the points of darkness still remaining in the world what with science and population growth and all and the focus on exploring the last frontiers.
Apocalypto: Blatant thumb in the eye to Mel Gibson aside, this book details either the coming of an apocalypse or the revival of society from a near-apocalypse. I don't mean shit like the Roman Empire collapsing; those aren't apocalypses because even though a lot of knowledge was lost and people ended up worse off, it's not like civilization went away. No, I'm talking shit like fantasy global warming or the zombie apocalypse gets out of hand. One part of the book will have a Grimdark tone to it which is pretty much your heroes determined to go down fighting or save their small sector of civilization. The second part of the book will be recovering from one of the aforementioned apocalypses; it's all Points of Darkness now and civilization is being to recover and make contact with pockets of survivors.
Awakening the Earth: So, everyone loves the Underdark. It's so goddamn popular that they made at least three D&D games which more-or-less featured the Underdark. But Underdark is a Forgotten Realms-ish concept. Whatever, we just won't refer to it. We WILL have a book that's all about the kinds of heinous shit that you can see underground like dorf fortresses and drow S&M pits and worse.
Heavenscape: People just LOVE ruins and temples that are decked out with golden fountains and ivory pillars and cherubic statues that piss out eggnog that never goes back. So we should have these. But how do you justify these things in a world where Steve the Crap-Covered Farmer gets called a ponce by his peers for bathing and changing his clothes too often? Simple, you put them in unreachable spots most people can't get to. Most of these things will be taking place on the heavenly planes (hey, blatant tie-in to Manual of the Planes), but there's no reason why you can't have these things in Dreamland, the Astral Plane, or even the frickin' CLOUDS.
Nightmare Necropolis: I know you want a book all about zombies and ghouls and ghosts and shit. Undead rock my billy socks and they're also really easy to write about, too. So they have the distinction of getting a A-tier book all to themselves when no other race does. Suck on that, dragons. Hey, remember earlier when I talked about not having an American West desert for the Silent Desert? Copypaste it here. You can have shit like skeleton coyotes and zombified purple worms and Indian Graveyards (retrofitted as not to offend fantasy sensibilities of course) and all that shit. Righteous.
Adventurer's Guild Log: Okay, have you been paying attention to your product for the past 5-6 years? You fucking better have, otherwise you need a new job. But regardless, THIS book compiles all of the most popular adventures and modules undertaken over the lifespan of this edition into a 'Greatest Hits'. This means that the most popular/contest-winning adventures at conventions goes in here. This means that this book will have a fucking Tomb of Horrors and a Red Hand of Doom adventure in it (one will focus on massively unfair Trial and Error Gameplay, the other on min-maxxing and rules mastery).