Simply D&D

4E was basically a different game than D&D. Like the other D&D-branded offshoots, such as the D&D Cooperative Board Games, D&D Parlor Games, or D&D Miniatures games, it was basically a D&D branded MMORPG-inspired tactical game. It was boldy and well crafted in that regard, but it just wasn't D&D, as such.

If Mearls and WotC want to craft other games which bear a D&D brand, but are actually different games than the TRPG, I'm all for it. But what I'd personally like to see, rather than an even more granular tactical game, would be an even *less* granular "D&D Storytelling Game" with a complexity akin to Hasbro's Tails of Equestria RPG. A fully-fledged, but ultra-streamlined TRPG experience, and not just for kids. Let's call it, say, "Simply D&D" or "Tales of the D&D Multiverse--A Storytelling Game."

Design parameters for this D&D Storytelling Game:
*It looks like D&D...if you blur your eyes. It uses all the poly dice, has 6 Ability Scores, HP, AC, Ability Checks, Saving Throws, and not much else.
*Possibly no math at all. Except for maybe adding and subtracting Hit Points. (Like Tails of Equestria.) In that case, SD&D would be a completely different "edition" than 5E, not directly compatible with 5E. *Another option would be to use the 5E Basic action economy, but rebuild the classes (and races and backgrounds) so that you get just one power per level. It still would not be directly compatible with 5E, but the action economy would be the same. (e.g. Move+Action. Action choices: Attack, Cast a Spell, Dash, Grapple, Shove, etc.) 
*The design parameters (mostly) stick to what has existed in some iteration of D&D. The game is not just a "fun" conglomeration of innovative tropes from OSR games. SD&D is not Dungeon World or Dungeon Crawl Classics. It is D&D.
* Ideally, research would compare how every single iteration of D&D resolved each subsystem, and pick the simplest and funnest*, with 5E Basic Rules as a default source. *(Note, the funnest is not always the verymost simplest; for example, the simplest weapon dmg was OD&D's 1d6 for all weapons. But it's more fun (and not too complex) to use differing weapon damages.)
*Also looks at Chainmail d20, Dungeon Command, DDM, and the D&D Cooperative Boardgames to see if there are simple (but fun) subsystems which can be used.
*Battles are designed to be resolved in possibly just a few rolls. Fights last no longer than the real time it takes for a typical fight scene in a film or novel to be resolved.
*PCs don't die when defeated, unless a story twist calls for it. Perhaps all PC deaths are consensual. Or maybe the 3 Strikes of 5E is good enough.
*Characters begin with only two or three powers (maybe: race/culture power, background power, and class power).
*Or perhaps say that only juveniles (of any race) are 1st level (and thus have only one Racial Power), and that every adult has a Background power (2nd level). And adventurers then gain Class powers at 3rd level.
*All character-build choices can be randomly rolled if the player wishes. Or hand-picked. So, there's an (optional) random table for all aspects character creation.
*Gain exactly one power each level. A spell is a power. By 20th level (20 sessions), a character will still only have 22 powers.
*A "class" is just a prebuilt template or tree of powers. But the player doesn't have to stick to any class.
*Option to start campaign before gain Class power (like DCC peasants/commoners), and so only have 
*Spell lists either trimmed to iconic Basic D&D list, and/or synthesized and somewhat abstracted like True 20 spells or DDM spells.
*No XP bean counting - just level up after every session. Or maybe, after every hour of play. Set a timer.
*Except for marquee items, treasure is abstracted as Treasure Level or something.
*Simply D&D is kinda like Lone Wolf or Super Endless Quest or DDM skirmish game, but still a TRPG.
*Except for special equipment, a party (or solo character) is assumed to have "coincidentally remembered" every kind of ordinary equipment necessary for the adventure. Including a 10' pole, if there is a hazard which calls for it. Its presence is retconned in the moment: "Hey, good thing I carried this 10' foot pole with us."
*No counting of rations, water, or arrows. Only starve or run out of ammo if a story twist calls for it.
*Encumbrance similarly streamlined. Maybe one Encumbrance slot per Str score, or something like that. There's a simple visual map on the character sheet showing the slots, and where the stuff is stowed: backpack, scabbard, belt pouch etc.
*Designed for a complete story to last no more than 1 or 2 hours. In fact, there could be a timer with the game to make sure! (Or maybe not.) :)
*All races, classes, spells, magic items, and monsters from all D&D editions also exist in the SD&D Reality, yet they are all super streamlined rules-wise. (E.g. compare 3.5E red dragon stats to BECMI red dragon stats. Both are red dragon.)
*Is presented as a comprehensive and legitimate "lens" on the entire D&D Multiverse, which is distinct from the "5e lens."  (Like how BECMI D&D was officially held to be a different "reality", wherein there were only four human classes and three demihuman races in the whole world.)
* However, unlike BECMI D&D, this D&D Storytelling Game would theoretically include every race and class ever seen in the D&D Multiverse (because it's fun to choose race & class, and to see new races and classes to chose from)...but each race and class would be streamlined into a much less granular "one power per level."
*Any class can be multiclassed at any level. Just choose Class Power from that class list and voila!
*Perhaps (like PF2), all or some racial/cultural powers can be chosen at any level, to form "multiraced/multicultural" characters. And justified by a retcon: "Hey, I just learned that my great grandfather was a dwarf. I'm going to start exploring that heritage."
*Setting wise, the classic dungeons of the D&D Multiverse are represented as 1-to-2-hour long "television episode / novella / film"-sized stories, completable in a single session. Even mega-dungeons are narratively condensed, where various key scenes of the iconic dungeon are played out, but separated by "cut scenes" which are just retroactively narrated in a quick paragraph of boxed text.
*World hopping from the start. Everyone is a member of the Adventurer's Guild (from AC1: The Shady Dragon Inn). (Like Pathfinder Society.) The Adventurer's Guild spans the entire Multiverse, including the more inaccessible worlds such as Athas. The Adventurer's Guild has planar teleportation (and chronomantic time-travel) resources to get the players anywhere, in any time. It is an assumed trope that the party is sent on adventures throughout the Multiverse, even from first level. That's just part of the game.
*As for products: the modules are printed in a small format (say, 7x10-inch). Could be softcover (cheaper) or hardcover (if Hasbro's business model requires a bit higher price point). Either way, the Simply D&D product line should be a lower price point.
*All modules are scalable from 1st level to 20th level (or possibly 36th level like BECMI or 40th level like 2e Epic), so that any series of Simply D&D modules could be used in any order of play. Has an appendix which gives scaled stats for every encounter. For example, the 1st level Tomb of Horrors, where Acerack is hardly more than a Skeleton with a Wizard spell or two. (Sacrilege?! Well, that's pretty much how the Lich of the Fantasy Forest gamebooks was portrayed. Same for the Skeletor-looking Lich which Strongheart and the Kids easily defeated on the way into Castle Venger in the D&D View-Master story.)
*Besides the usual dungeon crawl modules and wilderness modules (Isle of Dread), there could be a sort of World Sampler (or "Worldbook") module, which takes the characters on a whirlwind adventure to iconic sites throughout a single D&D World, such as a Mystara Worldbook module which gives the players a map of the whole world, and has a plot hook which takes them to a scene or two in Threshold, then to Castle Amber, to Alphatia City, to the Savage Coast, to the Hollow World, to the Immortal City of Pandius on the moon, etc...all in one session! And the module comes with  a "splatbook" appendix consisting of power options covering the unique races and classes of that world: "Hey, now that you've visited Mystara, you might pick up a level in Atruaghin Shamani or Darokin Merchant. Hey, and you discovered, when you picked up the Scent power, that one of your ancestors was a Lupin! Bow-wow!" 
*Make a Simply D&D module for each of the iconic D&D novels (Icewind Dale, Dragons of Autumn Twilight, etc...or maybe even condense a novel trilogy into a single session! The War of the Lance in 2 hours!). Module comes with Simply D&D stats for the iconic characters of the novel (Drizzt, Raistlin, etc), scaled to the difficulty level seen in the novel, or you can play it with your own character, and scale all the encounters up or down using the appendix.)
*Really tie together the timelines of the D&D Multiverse, and play on that in a clever, "meta" way, using the existing concepts of "paraverses" or "tangents." Like: "For this module, we're going to visit the Curse of Strahd timeline of Ravenloft. Next module, we'll visit the Classic Timeline of Ravenloft!" See:
*The "Simply D&D Reality" is slightly different than the advanced "D&D Reality" - in the SD&D Reality, an individual doesn't necessarily have all of their racial powers from the start. (This is similar to how monster PC (racial) classes were handled in BECMI D&D's Creature Crucible series. For example, in the SD&D Reality, not all Dwarves and Elves have Darkvision. And so, the D&D Multiverse is a bit different in this Reality, since Dwarven and Drow cities will all have lighting sources.
*There is an official, clear, and comprehensive conversion guide for all other editions of D&D, so that any adventure module from any era can be used for SD&D.
*Publish SD&D as Open Game SRD and DM's Guild. 

That's the kind of D&D i'd like to play. :)

Lastly, I'm glad Mearls mentioned his Nentir Vale home game. I was averse to Nentir Vale during the 4E era - not because it wasn't a thoughtful and innovative campaign setting - I rather liked the Points of Light theme and the synthesis of the various planes; but rather, because it purposely pushed out all other settings, and so it grated on me like FR. Yet now, I'd like to see Nentir Vale given the full campaign setting treatment, with a World map and everything - but as an equal peer of the other Classic worlds - Oerth, Mystara, and so forth.

Earlier notes:
Design parameters:

#1) Simply D&D (SD&D) is an authentic D&D experience. SD&D takes and synthesizes the simplest, most streamlined features from all editions of D&D...except...
#2) ...Where a feature is slightly more complex, but is more "fun" and "iconic", SD&D goes with that. (For example, OD&D had all weapons do the same amount of damage: 1d6, which is simplest, but we go with BECMI's slightly more differentiated weapon damages because it's more fun and iconic. And in BECMI, race was fused with class, which is "simplier", but choosing race and class is more fun and iconic.)
#3) SD&D is a vehicle for exploring the D&D Multiverse as a setting. The rules are greatly simplified, but the setting is not. SD&D is made to be used for exploring any and all adventures, from all D&D worlds and editions. The adventures and settings are converted to SD&D stats. 
#4) Though the the core SD&D book focuses on presenting only an iconic array of races, classes, spells, and magic items, all classes, races, and spells from all editions exist in this reality, but they are simplified and streamlined.
#5) The SD&D game is a different lens on reality than the other editions. It is a distinct and complete "paraverse" of the D&D Multiverse. (See

Question though: What about features which are more fun or more streamlined, but have not been featured in any edition of D&D?
-DCC's funnel method of character creation.
-start with only two powers: race power and class power. Then only one new power per level. (some similarities with True20, but even simpler)
-PF2's method of ability score generation.

Could be a straight adaptation of Tails of Equestria system. Which is not D&D in any form. But is simple.

(Earlier conceptions of SD&D):
Good article. As people have pointed out, there are some promising simple RPGs out there: Dagger! and One Die come to mind.

I started to sketch out a super-streamlined version of 5e here:

As a test run for 6e, I think Mike Mearls ought to take WotC's new My Little Pony RPG system, and strip it of its setting, and repackage it as totally complete game--I mean totally complete.

Design goals for this Simply D&D, The Storytelling Game:

Be able to advertise on the box: "We will never publish a rules expansion for this game. We will only publish Storytelling Adventures and new Storytelling Worlds."
•Explain that the Simply D&D game world is a different version of the D&D Multiverse than the 5E version. In the SD&D Reality, there are only four classes: Cleric, Fighter, Rogue, and Wizard--all adventures and classed NPCs in the whole Multiverse are represented by those four classes. And only the spells listed in the rulebook actually exist. BECMI officially used this "game reality" concept--see the old DRAGON magazine article:
•Make world-hopping (perhaps via the World Serpent Inn) the default framework for a Simply D&D campaign.
•As one Storytelling World, present the entire D&D Multiverse as a single setting, in a nutshell.
•Make sure the SD&D set covers every single rule or key rule expansion ever published for any addition of D&D--mass combat, underwater, planar. How? By boiling it all down into ultra-streamlined,
OD&D-style, hand-wave-based, but "official" rules. Make sure it's all covered: but "covered" could be a single chart, paragraph, or sentence.
•Give conversion guidelines for using Adventures from any edition. The goal is to Adventure, not to buy more rulebooks.
•Quickly release SD&D Worlds for all of Hasbro's key lines:
SD&D Magic: The Gathering, The Storytelling Game
SD&D Transformers, The Storytelling Game
SD&D GI Joe, The Storytelling Game
SD&D Candy Land
SD&D Monopoly. Hey, if you look at all of the spin-offs of Monopoly (Monopoly, Jr. etc) there is a story and setting behind it.
SD&D Clue
SD&D Mr. Potato Head

And for as many other IPs as feasible.
•Release the rules as an Open Game or Public Domain (Free Culture).
•Open the SD&D Hasbro settings to DM's Guild.

Voila! A blossoming new generation of RPGers.