For me, I'd leave 5E alone to let it continue to be published as-is. I'd like to go back to when we had both a (BECMI) D&D and AD&D line going at the same time.
So, I'd tackle B/X D&D, using mostly the Old School Essentials documents as the base. Put out a starter set that covers levels 1-5 for Fighter, Wizard, Thief, Cleric, Halfling, Dwarf & Elf, with a mini-campaign book (a 32-48 page brief compendium of Mystara with hometown of Threshhold) and an adventure (16-32 pages, PCs are sent to hunt down Bargle). Second product would be hardback that has rules up to 15th level, with additional option classes and option for separate race/class (i.e., Advanced Old School Essentials). Third product would be a revised/compiled Mystara campaign boxed set (or 3-book set, book 1 - Player's guide overview, book 2 - DM's in-depth compendium of the Known World, book 3 - (magic) items, monsters and one-page adventure ideas with mini-maps). Fourth product would be a campaign-length adventure (levels 1-15, lots of travel across Mystara to famous locations, at least one mass battle with the PCs at the head and an evil god [Zargon] behind it all). Fifth product would be a compendium of player & dm material - spells, magic items, joinable organizations and patrons, monsters and whatever other goodies will fit in the book.
Future years would be more adventures and converted campaign settings like Savage Baronies, Hollow World, Thunder Rift, Lost City (Cynidicea), Averoigne* (from Castle Amber), Blackmoor*, Horrobin (from Top Ballista) and hopefully a sprinkling of new places and ways to play in.
* would require licensing, but hey - I'm in charge to try, right?