Tenkar's Tavern, Greyhawk Grognard, JoetheLawyer, Diversity & Dragons, and Esper the Bard as well as a few others (G2 and Eldrad Wolfsbane) have made videos answering that question. I run the same thing through my head every few months (usually if I took over TSR instead of WotC back in 2000 but whatever). I don't make videos so I figure I'll just post. Assuming I was given the whole thing and carte blanche to control the product line without interference here is what I'd do, at least to start with.
D&D 2025, Spin off into a separate company with pretty much all the current employees (I'd let the employees pick and among those that want to stay with my 6E project I'd interview them and decide for myself). D&D 2025 project would have a new name and would live or die on their own. If Hasbro wants a My LIttle Pony or GI Joe version of a Role Playing Game this is the group they'd go to. This one would be all rainbows and characters never die and all that. Beyond that they can do as they will and good luck. Maybe it'll sell well, maybe not. That won't be my problem at that point.
D&D 6E. I'd take the 2014 edition and remove the fiddly abilities every level. Those are the things 'character builds' are made of and I don't want any part of builds. Instead I'd have a proficiency bonus equal to the PC level. Fighters get Proficiency in Attacks, Rogues get proficiency in skills. Some classes have their core ability (rage) and they'd keep these but lose spellcasting abilities. Clerics would get a re-work of Cleric Domains from 3.5 so a DM could quickly build a pantheon and have each deity meaningfully different. Also any naval stuff from The Saltmarsh adventures would be migrated into this book.
Starter Set I'd cut down the rules to the first 3 levels and bundle with The Keep.
The Keep I'd bundle this with the Starter kit and also sell it solo. This is (B1) In Search of the Unknown and (B2) Keep on the Borderlands. I'd update those two modules a bit and used the Keep as a base of operations for both. I'd also include interior maps and urban encounter charts for the businesses in the keep. The idea is a new DM has players explore Caves of Chaos until the DM feels comfortable enough to stock the Caverns of Quasqueton on their own. By the time they are done with both they should need no further help.
The Borderlands This would be the pyramid/Ziggaraut from B4 Lost City dropped into the swamp to the Southeast of the Keep, and B5 Horror on the Hill placed south of the River on the Keeps wilderness map (with the keep replacing Guido's Fort which was never detailed). This is not really a a follow-up as PCs should be high enough level to move on by the time they're done with B1/B2 but now you've got a large sandbox a DM could use again and again for starting players.
Lost Mines of Icespire Peak This would be both Lost MInes of Phandelven and Dragons of Icespire Peak. Put together to create a sandbox with a lot of adventures. Some of it would need to be reworked but the core is a decent adventure for low-level adventurers.
Other Adventures. I'm not gonna go over every module here. Just know I liked the way they bundled modules into a single book in Tales of the Yawning Portal. I'd do that as we updated the older modules. Basic or AD&D doesn't matter they are all being converted to 6E. If at all possible keep the modules in roughly the same tier of play or unified by theme/enemy. So perhaps X1 Isle of Dread (3-7) could be mixed with (I1) Dwellers of the Forbidden City (4-7) and (C1) Hidden Shrine of Tamoachan (5-7).
Campaign Worlds I'd keep the most popular (Greyhawk, Mistara, Ravenloft, Forgotten Realms, maybe Kara-Tur) and customize the rules and monster/class selection for each to make the game feel different in each setting. Every module we published would include a bit about where on each of those setting that module takes place.
Some settings (Planescape, Spelljammer, Dark Sun, Gamma World) I'd have updated to 6E and then released into the wilds as Creative Commons. Let individuals or companies take it from there if they want to create modules or expansions.
Lastly I'd return to others to their creators with my blessings (Eberron, Dragonlance, others),
Monster Manual I'd divide this into two or three books, broken up by tier of adventure. I'd go 4E style and have multiple versions of each monster (Slingers, Scouts, Shaman) so the DM has more to work with. I'd raid all the previous editions for content if necessary. I love monsters and might want a new edition every few years.
Dungeon Master's Guide I'd update the current 5E with the best stuff from every edition of DMG: 1E, 2E, 3E, 3.5E and 4E. If necessary split out the Magic Items stuff to be its own book.
Dragon Magazine I believe having a single source of info creates a sense of community that the websites don't. No Dungeon magazine, I'd makes sure Dragon was full of short adventures and lots of interior maps, and divide Deities & Demigods into pieces and have that as a regular column but specializing on the Cleric-side of things instead of statblocks for the Deities. Cover what was in Deities & Demigods and gods in different campaign settings and religions from novels. Doesn't have to be monthly, could be bi-monthly as long as its loaded with stuff.
