One D&D should open with the adventure: the Lost Mines of Phandelver.
I havent played it, am avoiding spoilers, but notice it has a good reputation. Many players and DMs rate it highly.
I want One D&D to have FOUR core books:
• Players Handbook: Human only, setting neutral, and ALL of the rules to play a complete D&D game.
• Forgotten Realms Guide:
− offers between 10 and 20 playable Nonhuman species according to their FR setting flavor.
− details the Neverwinter region, the main city and its hinterland, including the village of Phandelver.
− includes The Mines of Phandelver as a sample adventure, for characters of the Student tier (levels 1-4).
• Dungeon Masters Guide: cosmologies, worldbuilding, magic items, and robust playtested variant rules.
• Monster Manual: hostile statblocks, including traps.