Subraces! We've got 3 kinds of elf, dwarf and gnome, there should be plenty of room to make say, subraces for the explicit colors of dragonborn, for different demonic/devilish heritages for tieflings.
I'd like to see half-vampires and a legitimate aasimar race.
I was really disappointed with the genasi. The art certainly didn't help. Just felt like colorful people.
I'd like a plant race. Not like half-plants or something, but a legitimate plant people. Like Sylvari or Groot.
I'm also in favor of these being tied to settings to give them more interesting fluff.