Blizz has said they don't plan to add new races in Shadowlands and I think that's true.
Plus I think we can all agree it would be a bit silly for these members of the Afterlife to care about our petty mortal conflicts...
BUT we all know Blizz will shift design due to popularity, and so I wanted to make a few predictions of what they might do in a LATER expansion.
If Kyrians get popular... Probably blue skintones for humans (customization option)
...or Frost Vrykul get recruited to the Alliance with a heavy Stormheim / Bastion influence. (New/allied race)
If Maldraxxus is popular... probably new custom options for Forsaken
... or 'allied races' in the form of new types of undead (playable undead of other races or more extreme varieites of undead)
If Ardenweald's fauns and such are popular then either nelfs will get some of their characteristics (faun horns, etc)
...or they'll be made playable as 'redeemed satyr' or as revived victims from Teldrassil. (new/allied race)
If Revendreth is popular... probably belf customizations for San'layn or they'll go full allied race for em.
Possibly gargoyle stuff if they really take off but they're much less likely due to wing issues.