I would say this would also fits nicely under the general theme of “vertical development”
That being said some of your suggestions would require lore justifications. Think, for example, why would the Earthen, which are still as the titans intended them to be, so to speak, and generally friendly to both factions, all of a sudden appear in the alliance?
This though I wholeheartedly agree with, and never understood why it isn’t part of the game already, especially now that we have so much more tmog freedom:
Even something as basic as height adjustments, that essentially scales your character up or down, would add so much diversity. Obviously making sure that a “tall” gnome isn’t taller than a “short” dwarf for example.
I feel there should be more class specific customisations where applicable. Warlocks should be able to have certain corrupt/demonic elements on them (think Gul’dan): fel scars and markings, half demonic skin, glowing eyes etc. In similar fashion, druids can have nature driven elements, feral traits such as antlers, feathers, bear skin or even bark skin. Current DK and DH customisations could be expanded further. Shamans could get more elemental specific tattoos. I’m sure the list can be expanded further you get the idea.
I feel it’s not necessary for all classes to have special unique customisations, nor would it detract from their fantasies. Not sure what you would give unique to a warrior, hunter or rogue. More scar options? Body scars? Unique tattoos? Extra muscles for warriors? Not sure, and I believe it’s to some extend not needed since there isn’t anything particularly magical about them.
On the class theme, we could have more unique animations and emotes, even the idle ones. All the idle animations are basically the same, the character yawns and stretches their shoulders. Why not have mages play around with a small spec specific ball? Hunters could… You’re ready for this…? Pet their pets!!! I can see warlock’s imps pulling pranks on them, warriors inspect their weapons with pride and so forth!
Maybe there could even be voice and/or personality choices on the character customisation too! That would just change around the different voice emotes and various battle noises.
Just a series of small things like that which in turn make the big picture more exciting!