If they add a new race outside of the basic PHB list... my leanings go toward Shadar Kai (from Mordenkainen's Tome of Foes), as I think they as a race would fit into the lore well them coming from the plane of shadow. Plus I like the idea of a race with +2 Dex, +1 Con, superior night-vision and a teleport rechargeable on long rest as a build option for a more melee/ranged weapon themed class (Shadow Monk, Rogue/Assassin, or Warrior as examples).
That said if they introduced them, they would also likely need to introduce Eladrin. And it's not as if they have not already introduced one race from that tome of foes: Githyanki. So if they added the other playable races from the Tome such as Githzerai, Duergar, Eladrin, Sea Elf, Shadar Kai, and Deep Gnomes, it would not be too much of a stretch. After all they have the models for the Gith, Duergar, and Deep Gnomes already in place. And they would only need to tweak the elves a bit to fit the Eladrin, Sea Elves and Shadar Kai into the mix.
