For the Starter Set, they should stick to the 'classic 4' for each: Human, Elf, Dwarf, Halfling; Fighter, Cleric, Rogue, Wizard.
For the Core Rulebook, they can expand these lists somewhat. However, the choice of races, in particular, seems quite fraught. Therefore, they might be best "playing it safe".
If they can fit them in, I would go with: Human, Elf, Dwarf, Halfling, Gnome, Half-elf, Half-orc, Tiefling and Dragonborn all in the Core Rulebook. If they can't fit them all in, I would be inclined to drop back to the classic 4 again, and relegate everything else to supplements.
The one thing I would definitely do is drop the Elf/Eladrin split. Given that even 4e felt the need to have "elven subraces", I don't think baking that one into the rules is really justified. And having "Elf, Eladrin, Half-elf" as three of the eight core races was really not justified.
For Core Rulebook classes, I would suggest two of each role. My preferred set is probably Fighter (martial defender), Paladin (divine defender), Cleric (divine leader), Bard (arcane leader), Rogue (martial striker), Ranger (primal? striker), Wizard (arcane controller), Druid (primal controller?).
(It's not that I don't like the Warlord. I really do. But the Bard probably has a marginally greater claim to being 'classic'.)
Of course, if space permits, it would be no bad thing to be able to fit in the Assassin, Monk, Barbarian, Sorcerer, Warlord and Warlock as well - why not cover all the bases by including every class that has ever been 'core'? (Unfortunately, space won't permit, especially if they take the step of moving to a single Core Rulebook. Which would be wise, IMO.)