3E.
I'd keep all the trade dress and presentation. Underlying mechanics like BAB, F/R/W saves, and skill ranks would stay.
The system would keep the "Lego block" style multiclassing, and lean into it. 8-12 base classes, that all go to 10. Dozens of Prestige classes would be in the PHB, and the assumption is that almost all characters will multiclass.
A unified spells/powers known table across all classes. This would be the holding place for all class selectable abilities: wizard spells, warlock invocations, bard songs, martial feats and maneuvers, would all go in here. Some passive, some at-will, some on cooldowns or X/day abilities.
Class levels give some fixed features, advance BAB, Hit Die, and saves, and unlock new selectable option for powers/spells known. No feats by level, no ability progression except by explicit class feature or selected power.
A DMG with lots of standard guidance, unlockable PrCs, rules for making your own PrCs, and an in-depth magic item creation/treasure system. Wealth-by-level, and clear guidance that treasure into magic items is the expected secondary axis of progression.
Monster Manual with simple monster/NPC creation rules, but ones that parallel PC creation. Monsters have a Type (Giant, Dragon, Outsider), which corresponds to "class". One HD = one level, with corresponding increases in BAB/Saves. Each Type has a menu of selectable abilities. Every ability costs from 1 to X levels of progression. A 10th level Giant, for example, picks 10 points of Giant abilities.
Also has a "Non-combatant/Template" class, that gains levels but no Hit Die, and has its own ability menu. Allows for the 3 HP baker with 18 ranks in Profession: Baker.
After that, re-release the 3e FR book, lightly update to 1400-whatever DR, gloss over the last century. Make FR the core of the future releases. A few supplements a year, half gazetteer of a new area, half crunch associated (even loosely) with the region.