logically, the ones to come next should be half orcs, dragonborn and gnomes as they're currently missing but are PHB races. After that, anything would be cool - the easy route is to start including more of the various subraces for existing ones. There are a lot afterall (stuff like duergar, eladrin etc).
Tbh, outside of those, I'd personally rather see more of the subclasses added, there are a bunch in Sword Coast Adventurers Guide and then Xanthar's Guide to Everything too. Which also adds more racial customisation with new racial feats.
