I'm A Banana
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Man, so many people Agreeing On The Internet. 
I think part of that will be determined by the target page count, but circa 5-10 pages for each "chapter" hits a sweet spot. Some critters might have slightly more, some might have slightly less, but that seems to give a good balance.
I think the desire for minimal fluff is part of that desire for a traditionally formatted MM. Which, I guess, we avoid if we just call this third core book something different.
And I think even in such a book, it's necessary to make it very clear that DMs can take anything in their own direction.
This is presented as "one vision" of how goblins could be, presented so that DMs can tweak it if they want to. If you include goblins as wolf-riders allied with worgs in the book, but a DM decides he wants them on spider-back instead, it should be easy for him to either use the spider statblocks in the drow section, or just whip up their own spider statblocks from the DMG's monster-building directions. The kobolds might be dragon-worshipers, but it shouldn't take anything more than a change in how the DM describes them for them to be dog-rat-things. The tricks that DMs have been doing for 30 years with MM entries will still be valid tricks. What the MM says a dragon might be has never been the last word on what dragons are in D&D.
I think part and parcel of that is to have a monster-building system that is seamless and easy. 4e got us most of the way there, so they can probably handle it.
I think if I was to do this myself, I'd include "Goblinoids" as one entry, with goblins (minions), bugbears (brutes), and hobgoblins (soldiers) all there, working together. It's pretty normal in D&D to have these critters enslaved/conscripted/unified, anyway, and their differences will stand out more in contrast to the others if they're together.
Orcs would probably be a separate entry -- they're pretty distinct.
And swapping would just be a case of identifying equal-level monsters, much like it is in 3e or 4e (or even 1e and 2e, to a certain degree): any monster with a similar level would be a potential swap.
Creatures without their own lairs might get bundled into other creatures who have their own lairs. You might have wolves in with the goblinoids entry, rather than in their own entry, for instance.
Well, much like the MM, this book would be presumed to be full of DM information, not something the player needs to look at. Inevitably, that will happen to some degree, and for that, DMs can use the same tricks they've been using for years on players who read the MM -- refluffing, changing little bits, replacing, templating, etc. Just because the Kobold entry shows a trap in this room doesn't mean that there WILL be a trap in the room, especially if the players have already read the book.
For those, you'll need to divide them up into smaller, more easy-to-manage groups. The Demons entry probably wouldn't include ALL the demons D&D has available, but a selection of them relevant to most DMs with especially iconic demons (Vrocks, Marliths, Succubi), along with things like Demon Thralls and the like. Elementals, I think, are prime candidates to appear in other creatures' entries -- the Fire Giants summon Fire Elementals, for instance.
Sounds like a pretty good target to me. Usually the realities of budget and printing capability will dictate this for you, but if I could get "enough adventures to run characters from 1-10," that would hit the need of the 3rd core book pretty well. At 3 entries/level, that means 30 entries would be quite perfect.
If it takes most D&D groups about a year to get to level 10 anyway, we'd be able to then either publish a sequel, or write it off as a failed experiment.
That's certainly up to debate, though. The first Adventure Guide might want to hid a breadth of levels 1-20 so that multiple DMs at any given level can use it, and so 30 entries is just fine -- in fact, maybe a bit TOO many.
Let's see what I might do if I had one entry per level...hmm..
LV 1: Kobolds (dire weasels, urds, wild boars)
LV 2: Goblinoids (goblins, hobgoblins, bugbears, wolves)
LV 3: Feywood (elves, dryads, centaurs, giant eagles...)
LV 4: Orcs (orcs, trolls, ogres)
LV 5: Haunted Graveyard (skeletons, zombies, wraiths, ghouls...)
LV 6: Hag Coven (annis, shambling mound, displacer beast)
LV 7: Drow (spiders, driders, matriarchs)
LV 8: Medusa's Isle (Medusa, Siren, earth elementals, Gorgon, Basilisk)
LV 9: Glacier Peak (white dragon, frost giant, winter wolf)
LV 10: Pirate Seas (kraken, merfolk, water elementals, sahagin, sharks)
LV 11: Serpentine Swamp (black dragon, lizardfolk, crocodile, catoblepas)
LV 12: Caverns of Madness (mind flayer, intellect devourer, mimic)
LV 13: Living Jungle (treant, green dragon, yuan-ti, naga, hydra, snakes)
LV 14: Vampire's Castle (vampire, ghost, banshee, werewolf)
LV 15: Distant Sands (blue dragon, wind elemental, efreet, janni, mummy)
LV 16: Primordial Depths (beholder, oozes, gibbering mouther, grell, aboleth)
LV 17: Crimson Volcano (red dragon, salamander, fire elemental, azer)
LV 18: Shrine to Graz'zt (balor, marilith, vrock, succubus)
LV 19: Shrine to Asmodeus (Kyton, Gelugon, Pit Fiend, Erinyes)
LV 20: Return of the Terrasque (terrasque, apocalypse cultists, golems)
...which isn't a bad quick survey. There's a few left out, but that is the nature of the design, here. Not gonna be able to fit in every tabaxi and rakshasa. And there's still probably room for a few more, especially short entries like ooze hazards and the like.
Rust Monsters and Black Puddings and Displacer Beasts are good candidates, I think for "support monsters." Maybe the gnolls use Displacer Beasts as guardians in their camps. Maybe Rust Monsters and Black Puddings live in the Abandoned Mines along with some undead and earth elementals.
Mindflayers and Beholders probably are anchor-monsters, with the former having an assortment of psionic minions, and the latter being good candidates for other alien aberrations like gibbering mouthers.
Again, this is subject to discussion.

steeldragons said:Now...what monsters go in/lairs get made? How many pages each? How many creatures are balled into each preset environment?
I think part of that will be determined by the target page count, but circa 5-10 pages for each "chapter" hits a sweet spot. Some critters might have slightly more, some might have slightly less, but that seems to give a good balance.
steeldragons said:And there's the question of fluff...which many posters here, at least, seem to want as minimized or generalized as possible. Are we going to have a kobold warren of dozens of little dragon-worshippers and half-dragon-kobold sorcerers with a "young" blue dragon at the end...or red...or green dragon? Or a warren of little scaly dog-men? Either presenting with their traps and, seemingly all important vermin. And how are you going to argue/survive the backlash of one preference or the other? Are goblins automatically "wolf/worg-riders"? What about giant bats or giant lizards? Or are we going to have "Forest Goblin", "Subterranean Goblin", "Dessert Goblin" "Tundra Goblin" entries...spread willy nilly throughout the book with fifteen entires in the, extraordinarily important for this work, "Index"?
I think the desire for minimal fluff is part of that desire for a traditionally formatted MM. Which, I guess, we avoid if we just call this third core book something different.

And I think even in such a book, it's necessary to make it very clear that DMs can take anything in their own direction.
This is presented as "one vision" of how goblins could be, presented so that DMs can tweak it if they want to. If you include goblins as wolf-riders allied with worgs in the book, but a DM decides he wants them on spider-back instead, it should be easy for him to either use the spider statblocks in the drow section, or just whip up their own spider statblocks from the DMG's monster-building directions. The kobolds might be dragon-worshipers, but it shouldn't take anything more than a change in how the DM describes them for them to be dog-rat-things. The tricks that DMs have been doing for 30 years with MM entries will still be valid tricks. What the MM says a dragon might be has never been the last word on what dragons are in D&D.
I think part and parcel of that is to have a monster-building system that is seamless and easy. 4e got us most of the way there, so they can probably handle it.

steeldragons said:Are there going to be entries for Goblin Camp, Hobgoblin Camp, Orc Camp, Bugbear Camp...or doing one with some DM's Note that you can swap out the creatures as desired? All of those "cultures" are to be expected, by players, to be one and the same? Or, Orcs and Hobgoblins can be swapped...Bugbears and Ogres can be swapped...and where are the stats for those that don't have their own lairs presented?
I think if I was to do this myself, I'd include "Goblinoids" as one entry, with goblins (minions), bugbears (brutes), and hobgoblins (soldiers) all there, working together. It's pretty normal in D&D to have these critters enslaved/conscripted/unified, anyway, and their differences will stand out more in contrast to the others if they're together.
Orcs would probably be a separate entry -- they're pretty distinct.
And swapping would just be a case of identifying equal-level monsters, much like it is in 3e or 4e (or even 1e and 2e, to a certain degree): any monster with a similar level would be a potential swap.
Creatures without their own lairs might get bundled into other creatures who have their own lairs. You might have wolves in with the goblinoids entry, rather than in their own entry, for instance.
steeldragons said:How do we defend from the point I've made repeatedly that once you've used one of these little scenarios/environments your players now have a map (mental or literal) of what any encounter with a camp of goblins or orcs is set up like...i.e. using each piece of this adventure guide more than once.
Well, much like the MM, this book would be presumed to be full of DM information, not something the player needs to look at. Inevitably, that will happen to some degree, and for that, DMs can use the same tricks they've been using for years on players who read the MM -- refluffing, changing little bits, replacing, templating, etc. Just because the Kobold entry shows a trap in this room doesn't mean that there WILL be a trap in the room, especially if the players have already read the book.
steeldragons said:Don't even get me started on how/where to build an encounter/lair/adventure that incorporates the width and breadth of Demons or Devils...Elementals...Giants...One entry for Hill Giant Steading, one Frost Giant Glacier, and one Fire Giant Volcano, perhaps? What about the others? Should Treants be listed/treated as "Wood Giants" or have their own place in some "The Ancient Forbidden Forest of the Faeries" setting?
For those, you'll need to divide them up into smaller, more easy-to-manage groups. The Demons entry probably wouldn't include ALL the demons D&D has available, but a selection of them relevant to most DMs with especially iconic demons (Vrocks, Marliths, Succubi), along with things like Demon Thralls and the like. Elementals, I think, are prime candidates to appear in other creatures' entries -- the Fire Giants summon Fire Elementals, for instance.
steeldragons said:How many places...the "anchor monsters" I think you've called them, are we presenting in this "Adventure Guide"? And how big are we talking? 3 or 5 or 10 pages per "entry"? A simple 150 pages? 200? 400? (at an average 5 pages per entry, with each individual monster's stats, ecologic/interactive fluff, maps, traps, etc. doesn't sound unreasonable, 150 pages gives us 30 "encounter/lair/mini-adventures."
Sounds like a pretty good target to me. Usually the realities of budget and printing capability will dictate this for you, but if I could get "enough adventures to run characters from 1-10," that would hit the need of the 3rd core book pretty well. At 3 entries/level, that means 30 entries would be quite perfect.
If it takes most D&D groups about a year to get to level 10 anyway, we'd be able to then either publish a sequel, or write it off as a failed experiment.

That's certainly up to debate, though. The first Adventure Guide might want to hid a breadth of levels 1-20 so that multiple DMs at any given level can use it, and so 30 entries is just fine -- in fact, maybe a bit TOO many.

Let's see what I might do if I had one entry per level...hmm..
LV 1: Kobolds (dire weasels, urds, wild boars)
LV 2: Goblinoids (goblins, hobgoblins, bugbears, wolves)
LV 3: Feywood (elves, dryads, centaurs, giant eagles...)
LV 4: Orcs (orcs, trolls, ogres)
LV 5: Haunted Graveyard (skeletons, zombies, wraiths, ghouls...)
LV 6: Hag Coven (annis, shambling mound, displacer beast)
LV 7: Drow (spiders, driders, matriarchs)
LV 8: Medusa's Isle (Medusa, Siren, earth elementals, Gorgon, Basilisk)
LV 9: Glacier Peak (white dragon, frost giant, winter wolf)
LV 10: Pirate Seas (kraken, merfolk, water elementals, sahagin, sharks)
LV 11: Serpentine Swamp (black dragon, lizardfolk, crocodile, catoblepas)
LV 12: Caverns of Madness (mind flayer, intellect devourer, mimic)
LV 13: Living Jungle (treant, green dragon, yuan-ti, naga, hydra, snakes)
LV 14: Vampire's Castle (vampire, ghost, banshee, werewolf)
LV 15: Distant Sands (blue dragon, wind elemental, efreet, janni, mummy)
LV 16: Primordial Depths (beholder, oozes, gibbering mouther, grell, aboleth)
LV 17: Crimson Volcano (red dragon, salamander, fire elemental, azer)
LV 18: Shrine to Graz'zt (balor, marilith, vrock, succubus)
LV 19: Shrine to Asmodeus (Kyton, Gelugon, Pit Fiend, Erinyes)
LV 20: Return of the Terrasque (terrasque, apocalypse cultists, golems)
...which isn't a bad quick survey. There's a few left out, but that is the nature of the design, here. Not gonna be able to fit in every tabaxi and rakshasa. And there's still probably room for a few more, especially short entries like ooze hazards and the like.
steeldragons said:Where's the Rust Monster go? Displacer Beasts? The Mindflayers? The Beholders and Black Puddings? If this is to be the 3rd core book, then the D&D iconic monsters need to be there (at least a good chunk of them)...what's that list look like? Is that doable in 30 entries? 50? 100?
Rust Monsters and Black Puddings and Displacer Beasts are good candidates, I think for "support monsters." Maybe the gnolls use Displacer Beasts as guardians in their camps. Maybe Rust Monsters and Black Puddings live in the Abandoned Mines along with some undead and earth elementals.
Mindflayers and Beholders probably are anchor-monsters, with the former having an assortment of psionic minions, and the latter being good candidates for other alien aberrations like gibbering mouthers.
Again, this is subject to discussion.