if they released, say, a Dragonlance sourcebook, then they immediately reduce the number of potential sales to only the subset of players who want to use Dragonlance. They want every book they release to be tempting for every player. Fragmentation of the market through numerous campaign settings was partially responsible for the great RPG crash.
Yes, tat's been the conventional wisdom ever since Ryan Dancey first voiced it after the fall of TSR. And it must be more than half true. But all truths have their limit. Mearls and team know that maxim well, yet they still ventured to publish two 4E Dark Sun books, and two 4E Eberron books, three 4E D&D Gamma World boxed sets, and a Ravenloft boardgame. What they did do, was remove the distinctive logo and branding, so it all looked like a single product line.
5E's Curse of Strahd doesn't have the Ravenloft logo on the cover either.
In my OP I offer a way to refrain from fracturing the fanbase, while at the same time, fully tap into the vast array of D&D Worlds. This is a totally different gesture than 2E or 4E or 5E (so far).
The 2E gesture was: Let's publish a bunch of differently branded worlds which each develop their own separate, fragmented clientelle. That's what you and Ryan Dancey are pointing to as a flawed strategy.
The 4E gesture was: Let's cram the best bits of all the D&D worlds into a new patchwork world called Nerath, and hope everyone forgets about the other worlds. But then, after everyone is (hopefully) hooked on Nerath (which didn't happen) we'll release a few unbranded books for other worlds (FR, Eberron, Dark Sun).
The 5E gesture is better thought out: We'll make FR the default world, and release other settings very slowly, and present them as one-book rules expansions or iconic adventure sites (Castle Ravenloft). We'll mention all the other worlds in the DMG, to affirm that they do exist, and give suggested placements of each adventure for each world. (Not a bad strategy.)
I'm not talking about reverting to the 2E strategy.
I'm suggesting sticking with the basic 5E strategy, but then building from there in a way that's never been done before.
I'm suggesting to recast the entire D&D Multiverse as a single brand, and to amp up the presence of planewalking, spelljamming, and chronomancy, so that many of the next generation of D&D groups are accustomed to leap from world to world, even at low levels (via an expanded network of World Gates), exploring any and all of the D&D Worlds - in a similar way that 5E parties were "expected" to journey (via the Mists) to Ravenloft for the Curse of Strahd adventure.
This would be accomplished by:
1) A rules expansion which focuses on the three "transitive" modes: planewalking, spelljamming (space travel), and chronomancy (time travel). Time Travel makes available for exploration such rich settings as the Age of Blackmoor (of Mystara), the Arcane Age (of Forgotten Realms), and (future) Greyhawk 2000, and all other past and future events as depicted in old D&D and AD&D adventures.
2) A series of epic "Crisis on Infinite Earth"-style novels wherein Drizzt teams up with a team of heroes from all worlds (Tasselhoff from Krynn, Aleena the Cleric from Mystara, and so forth), in order to save the Multiverse from a conspiracy of villains and evil gods from all worlds. This book series would be branded as Forgotten Realms and Dragonlance and D&D at the same time. With all three brands on the cover.
3) The publication of a "Grand History of the D&D Multiverse". A compiled timeline of all D&D Worlds. Similar to the "Grand History of the Forgotten Realms" which came out a few years ago.
4) The publication of an "Atlas of the D&D Multiverse" which features brand new, complete world maps for *all* of the key D&D Worlds, with planar and spelljammer route maps to each.
5) Open up all of worlds for amateur-publishing through DM's Guild. Then we're rolling.
See, it's not that much. Just four books. Then we'd have a truly shared, complete D&D Multiverse setting and brand.
