What I want in core
Races: Dwarf, Eladrin (not necessarily fey walking), Elf, Gnome, Halfling, Half-Elf, Human + one more (something that fits more with oD&D-3e core (e.g., lizardmen, half-orc, half-ogre (or goliath) rather than Tiefling or Dragonborn).
Classes:*
Barbarian, Bard, Cleric (non-martial priest), Druid, Fighter, Holy Warrior (Paladins and martial clerics), Ranger (non-spellcasting), Rogue, Shaman, Warlord, Wizard
Introduce new PC races in their own book. If they exist in real world cultural myth and legends, cover that including a write-up. Then, give a base race write-up based upon biology leaving out the cultural elements (e.g, racial enemies, tactics, weapon proficiencies) and explore how they might be altered for different environments (arctic, desert, swamp, urban etc.) and campaigns- here is a place to give suggestions of cultural elements. Next, if they existed in previous edition settings cover those. Finally, give a generic cultural version or two that might be plugged into any setting.
Introduce new classes in their own book. Give an in-depth treatment. If sorcerers examine the different types of bloodlines (Fey, Dragon, Infernal, etc.). Do the same for Warlocks.
Look at real world cultures, literature, film and show how to capture these versions different versions rather than trying to come up with the one iconic "D&D" flavor.
And, if the class involves spellcasting or psionics- hire Steve Kenson to write it an design the mechanics. He did excellent work on the Shadowrun magic books for 1e-3e, Green Ronin's Handbooks for the Psychic, Shaman, and Witch. Nothing WOTC has ever done has, in my opinion, even come close.
* I am willing to have less classes
Adept (artificers, non-warrior priests, druids, shamans, sorcerers, warlocks, witches, wizards)
Expert (Sages, thieves)
Warrior (Barbarian, Fighter, Knight, Ranger (non-spellcasting)
Warrior Adept (Holy Warriors, Martial Clerics, Paladins (spellcasting), Ranger (spellcasting), Warden)
Physical Adepts (Monks, Ninjas (mystic powered)
Something blending the Expert and Warrior (Swashbucklers, Non-mystical ninjas, non-mystical unarmed martial artists, and other highly skilled, but lightly armored warriors would belong here. maybe Ranger (non-spellcasting) and certain "barbarian types" would be here instead of Warrior)
Then with backgrounds/themes, skill selection, talent trees (and/or powers), etc. you could tailor to your concept.