I find some of the ideas in this thread very strange.
I do not think there are very many DMs who care so little for designing (and modifying) traps and monsters and such that they would be perfectly happy with an advice-only DMG. I think virtually everyone would want the DMG with the crunch in it. If anything, I think the practical effect would be experienced DMs buying only the crunch book because they already know how to DM, and less experienced DMs getting confused or frustrated by the two different books for DMs.
I can understand the impulse to break things up in to as many books as possible--indeed, I agree that it would be very convenient--but every new hardcover book drives up the price. Three core rulebooks would probably cost $35 a piece for $105. Five core rulebooks with the exact same amount of content (for example; maybe a book of rules, a book of players options, a DMG, a magic item book, and a monster manual) would probably still have to cost
at least $25 a piece, for $125 total.
What's more, I don't think we want enough magic items in the core to fill an entire book. I know I don't want to have to pay for an entire book's worth of magic items (when 50 pages or so will do) to invest in core.
I
really don't think there's any way all of the core material could fit into a single 300 page book. 5e is trying to accommodate all different sorts of players. That means all kinds of races and classes included in the core: gnomes and barbarians, vancian and nonvancian arcane casters, at the very least, right in the core. It also needs reasonably tactical rules for the tabletop for those who like that style. And all of D&D's iconic monsters. And good DM advice. And full rules for designing and modifying monsters and traps. And a good selection of magic items. And themes. And on. And on. There's no way; the days when D&D could fit in 462 pages (let alone 300) are just over.
As for me, I don't really have any ideas for modifying the three-book structure. I would, though, warn against trying to make the core books too noob friendly. D&D needs to be noob friendly.
But the core rulebooks should not worry about being too noob friendly. A really good introductory box set should worry about being noob friendly. Now, don't get me wrong; the core rulebooks need to be readable and clear. But they shouldn't be introducing the game to brand new DMs or groups; WotC needs to finally make a good intro set.
Once again:
Pathfinder Beginner Box. Pathfinder isn't better than 4e. But its starter set annihilates every intro set Wizards has ever made combined. WotC has never made a good intro set. The 3.0 Adventure Game was terrible. The 3.5 Basic Box was terrible. The 4.0 Starter Set was terrible. The Essentials Red Box was, you guessed it, terrible. They're all terrible. It's no wonder Wizards can't get new players into the game.
The Pathfinder Beginner Box is amazing. It has pregenerated characters and character creation rules (how anyone could possibly think an intro set could be acceptable without both those things is beyond me). It covers five levels.
Five. It comes with two absolutely beautiful books packed with content. It comes with stellar standup cardboard pawns. Paizo
buried WotC on this particular issue.
5th edition's beginner box must, absolutely must, be orders of magnitude better than any of the dreck WotC has foisted on new players before. Wizards, if you're listening, look, look at the Beginner Box. Do that. Exactly that. Your starter sets are an embarrassment. No matter how well-designed your new edition is, it
will fail without a good starter set. You need to make a starter set that covers four or five levels, with pregens as well as full character creation rules, that's absolutely packed with content. If you do not do this, you will keep failing to attract new players.
And it needs to come out on day 1. Right alongside the core rulebooks. Do it, WotC. You'd better, for your own sakes.