A slightly blueish/grey variant of a race that already has 2 to pick from.
You know which race would have been cooler?
Ethreals.
Great backstory, can be both Horde or Alliance, they look cool and their humanoid proportions would suit all equipment sets.
Their transmogs would be mint 
We already do, they are called void elfs??
Elves? Reskinned blood elves??
Nah nah nah
The real master race
Ethereals would have nothing to do with the setting or the expansion lore though…
Now Naga on the other hand… flooded Subterranean tunnels and islands? Woulda been a nice time to introduce them as playable.
I do like the idea of playable Ethereals though. Just don’t think this was a good expac for it.
I mean… welcome to WoW? Blizzard has been selling low-budget recolors as “new” races for a while now.
At least Vulpera were actually a new race, even though they are based on an old model. Dracthyr can be somewhat counted as a new race too, even though it’s a reskin. Half counted because there’s only one body type.
But other than that, the devs have been producing this cheap stuff for quite some time now. It is what it is and you will even find white knights, who will defend a third low budget dwarf clone. 
What about no other allied race and just improve existing races, update textures of their heritage armor, more customizations etc?
Oh no. New race means race changes, new boosts, etc.
I agree. We could have so many cooler races. Merlocs, Gnolls, Harpies, Valkyr, Gilgoblins, Those walrus people, Sethralak, Naga, I know most of these aren’t thematic but come on. :o but no, a 3rd form of Dwarves. We got base, and now Dark Iron, and now Earthen.
World of Dwarfcraft
Remember when people used to whine there were too many Orcs? And now somehow we have three Dwarves. When one Dwarf race was already pushing it.
No. Give us the races we’ve been asking for for literal decades then we can worry about focusing on polish for a while.
Well then, enjoy the Earthen I guess.
Personally, I’ve never understood the hype behind Ethereals. That said, I’m certainly not over the moon with Earthen. I certainly won’t be playing one.
Personally, I think there are really only two outstanding races that could actually add something to their faction.
Ogres for the Horde. They’ve been missing from the roster since Warcraft II, they’d go a long way in cementing the Horde’s identity (Currently, it feels less like a coherent faction and more like a halfway house for undesirables). Canonically they’re already in the Horde, etc.
For the Alliance, I’d really like to see Furbolgs. Blizzard has consistently said that they wanted a more “Monstrous” race in the Alliance. Going by old Dev statements around the time Draenei & Worgen were added, this is part of the reason why they chose both of those races. Unfortunately, Worgen feel very contrived & Draenei just don’t deliver on the premise very well. Furbolgs are “monstrous” and “savage,” but overall gel with the Alliance in a way other prospective additions don’t. They’re also already canonically allied to the Alliance and have very long-standing ties with the Night Elves.
Beyond that? Arguably Forest Trolls for the Horde, what with the Revantusk, but I don’t think we need much beyond that.
After getting those obvious odd men out into factions, it’d make far more sense to expand racial customization in my opinion. In light of Dragonflight customizations, certain allied races feel VERY simplistic and limited.
Personally, I think Blizzard needs to get over their fear of overlap, and just allow Allied Races & their main race to share customization whenever possible.
Take Mag’har and Orcs, for example. There’s really no reason why the Mag’har shouldn’t enjoy every customization that regular Orcs have (Grime, scars, expanded eye colors, all hair and facial hair styles, etc.) save for arguably the Red Eyes of MU Orcs. The same applies vice versa. (Also, both should enjoy NPC options. Mag’har Orcs should enjoy the Blackrock WoD skin customization that features no burn scars, and the WoD Blackrock eyecolor that doesn’t turn their ENTIRE eyeball orange.) In practice, it’d be nice to make an Mag’har with Grime & Stubble, or an MU Orc with the Grom Hellscream hair.
Similar cases could be made for Mechagnome hairstyles & Facial Hair. Dwarves & Dark Iron Dwarves (Dark Iron Beards are still tied to their mustache, whereas regular Dwarves have no such limitations), Draenei & Lightforged Draenei, etc.
This would allow a whole host of new customization options, certainly more valuable than the “Blue and muted green” hair color options we’ve been given recently, and frankly it would take even LESS time.
I’m excited to learn about the earthen.
Dark iron is the one that shouldn’t have been a thing. Should have just been an option for regular dwarves… Should have got sethrak instead.
Why? Dark Iron are actually a part of the Alliance. Sethrak just have a “unqiue” model.
Why? Because that’s what I wanted.
Kul Tiran could do with some love to hair styles, tattoos and beards
Even an option to reduce the gut a little. Even ogres have abs
But I really want gnolls. They are cute, easily neutral. :o Or harpies. Lets get more neutral races!
We could stand to lose some Elves too.
The new Gnoll models are so nice! Would love to see them added over most suggestions.
Seeing how The Harbinger quest set the Ethereals up as some of Xalatath’s prime minions, and reintroduced Locus Walker back into the story for the first time since Legion, I’d say they have something to do with the story and setting.
Just about anything would be more interesting than the earthen but that dead horse has been well beaten by now.