I’m probably a bit biased stating this, but it still has some truth to it: The most popular races that have been neglected customization-wise should be focused on 1st, followed by the less popular races.
So by order of popularity and lacking customization:
Worgen: They need more hairstyles, facial expressions, and tails.
Void elves: They need a lot of hairstyles, as well as tattoos, voidy hair effects, jewellery, voidy skin tones etc. There’re threads up listing everything that’s requested.
Dark Iron Dwarves: They probably have the least customization out of all races atm. They need skin colours, hair colours, hairstyles, and jewellery.
Kul’Tirans: They should share all human skin tones, should get more hairstyles, and more faces. They should also be able to be Paladins.
Zandalari Trolls: They need aztec style gold jewellery, more tusks options, more skin colours, hairstyles, faces etc
Blood elves: As they’re the most popular and have been making requests for several years now, they’re making it to the list. They want tattoos and face runes, scars, red eyes, and undead skin tones. There’re Megathreads up listing their requests.
Nightborne: They badly need some lighter skin tones, more youthful, non-wrinkly faces, and more prominent tattoos.
Vulpera: Need more fur patterns, facial expressions and more snouts.
Pandaren: They need lots of jewellery, facial expression options, hair colours etc.
Honorable mentions: Draenei have no eye colours…neither do LFD.
I didn’t plan to list all races but damn, that’s 9 races in dire need of stuff right there.