I prefer D&D being a game engine that can handle completely different settings.
So keep the core rules of the Players Handbook customizable, broad, openended, and suggestive.
That said. It is important to have a narratively rich setting. So far, there are three core rule books: Players Handbook, DMs Guide, and Monster Manual. There should be a fourth core rulebook, a setting guide.
• Players Handbook: ALL (!) of the rules necessary to play a game of D&D. The only book necessary.
• DMs Guide: running adventures, encounters, variant rules, worldbuilding, magic items, wealth, advice.
• Forgotten Realms Cyclopedia: detail flavors, maps, species, cultures, class organizations, etcetera.
• Forgotten Realms Monster Manual: monster stats with how their ecologies fit into the Realms.
These are the four core rulebooks.
Supplements include Eberron Cyclopedia, Eberron Monster Manual, etcetera.