The only people who should be making DND 6e are us in this site.
It's a heavy burden, but one I would gladly undertake.
Mainly I expect I'll be arguing with everyone else here for why we need a Weapon vs. AC matrix at least as an optional rule.
I do wonder how I'd go about writing a 6th edition, if I actually wanted it to be succeed.
Let's be real: a full on retroclone would not go over well with D&D's current market. I think you're on to something though, in that the way to go would be to supe up the DM's guide with tons of optional rules. Outside of the politics and culture around it (which you couldn't fix just in the core rulebooks), 5e's biggest problems seem to be with what you might call "misplaced complexity". Too many class permutations and too much action economy, and not enough world/exploration/dungeoneering rules. It honestly blows my mind that D&D still doesn't have chase rules in the PHB.
I forget who originally made the suggestion, but at this point I think WOTC would be smart to split D&D off into Basic and Advanced lines again. Then they could satisfy both the people who just want to "tell stories" and the people who want the whole immersive sim element, and probably sell twice as many books. Hell, they could probably split the Advanced line off into separate low and high-fantasy lines and sell even more.
I don't really understand why WOTC is so concerned with making D&D a monolith, when every other major company with a successful ruleset (Chaosium, Paizo, Pinnacle, Modiphius etc.) instead focuses on making multiple games with similar rules. If they had multiple game lines, they wouldn't have to keep changing the rules of D&D to sell new products.
Shit WOTC, you already own the rights. Make a designated MTG RPG based on the 5e ruleset, and you'll probably be printing money off of it for a decade....
EDIT: The more I think about it, the more convinced I am that going down the SWADE/GURPS route of making D&D the basis of a generic ruleset for a bunch of licensed settings would be a huge money-maker for them. They have the market profile, and Hasbro has the money to outbid everyone else for licenses. I don't think it would be a good game, and I'm not hoping they do it, but I bet it'd work.