Here's what I'd like to see Mike Mearls and WotC do:
In regard to Rules Complexity:
For 6E, turn D&D into a game which basically no more complex than The Black Hack. Really. Start with only one Power (spell, proficiency, class feature, feat or racial trait). Turn everything into simple "Skills & Powers". It doesn't even have to be "balanced." Gain one Power per level. Super simple. But not white bread generic like the existing Basic Rules PDF, which reads like software code. And make all classes, races, spells, monsters, and items available, but super simplified. Really juice up the Power descriptions and titles, to match what characters can do in D&D novels.
In regard to Lore Density:
But at the same time, release a complete series of gigantic hardcover compendium books from star, which include literally all spells (Spell Compendium), all magic items (Encyclopedia Magica), and all monsters (Monster Manual, ala Echohawk)...FROM ALL EDITIONS. From the start, like in the first year of 6E release. Along with an Atlas of the D&D Multiverse with world maps for all previously-published D&D worlds (ala Karen Wynn Fonstads' atlases of Krynn and Toril). And a Timeline of the D&D Multiverse (ala Brian James FR work), synchonizing all the worlds' timelines, and getting them all up to speed. And a Who's Who's of the D&D Multiverse (aka Shady Dragon Inn) with all iconic characters and NPCs from all editions, from all D&D modules, novels, comic books, boardgames, and videogames...Bargle, Drizzt, Aleena, Regdar, Tasselhoff, everybody!
And drastically lower the prices of the Classic PDFs, and push them as bundles which cover *everything* ever written about each D&D World. So newcomers can brush up and become loremasters without breaking the bank.
And at the same time, make a sandbox-style Worldbuilders Guide the default campaign...a sort of "un-setting."
The streamlined game + accessibility of the lore, pleases beginners and grognards, sandboxers and loremasters, at the same time!
But if really want a super-complex iteration too, then also have an Advanced D&D 6E, and support both BD&D 6E and AD&D 6E with dual-stats in all products. Ideally, they. But keep them separate.
Furthermore, to grow the player network even more, I'd suggest the sacriligeous but humble route of WotC simply adopting Pathfinder Second Edition whole cloth for AD&D 6E. Just relable it D&D 6E and add in the "missing" iconic D&D races/classes (including 1.5E Unearthed Arcana): Tiefling, Dragonborn, Svirfniblin, Drow, Warlock, Warlord, Cavalier, Acrobat.