What would it be in the game? It can be anything, get creative!
For me personally, i always loved Witcher 3 and how you were able to put trophies on your horse. I would totally love if you could carry dungeon/raid bosses’ heads, items as trophies on your mounts. This would be another way to tell if someone killed a boss or not.
Share yours! Character, mount, any customization goes
Reminds me of these FPS games where you can hang stuff on your weapon, like cheese… seriously who wants a cheese smelling gun?
I liked the whole gear dye system in GW2 but I feel like it would take from the fantasy of certain items so maybe not a good idea for WoW.
Casting animations with staves / wands, if that counts.
Or weapon enchant effects applied to hunter auto attack arrows.
Oh you mean warzone where there are stickers, etc for weapons? I agree, i wouldn’t like that in wow, but in Witcher 3 u were able to carry those thropies once u killed those monsters, like a gryphon head.
I don’t see why that wouldn’t be nice, it certainly wouldn’t hurt to have it.
I like it.
Right? Some of the most basic horse mounts could be interesting aswell.
Even for raid bosses that exist in the game for a long time. A small scale of Deathwing that would be still smoking, like a hot lava rock on the side of your mount, or a demon skull, whatever basically.
Red and green eyes for Deathknight
Frost, Unholy, Blood.
Blonde hair for velves, like Alleria
And San’layn (either as a new race, or extra customization to blood elves)
Something that can be implemented easily into WoW… scaling for overall size of character, that’s already done in game through a whole bunch of toys, buffs and such since Vanilla.
Who wouldn’t want to play as a midget Tauren or a Human-sized Gnome? 
How would your little Alpaca, for example, carry Onyxia’s massive head? 
Long beards for Humans! #LBFH
That would be really nice, stuff like you see in HP, or Gandalf, etc.
That reminds me, when Gandalf is fighting the Balrog IIRC he uses the sword to cast a spell too, that would also be cool for the casters that wear swords.
Different voice packs for different race/classes.
Different animation sets, likr why does every undead warlock cast chaos bolt the same way for example, same for meeles.
Spell colors and themes simmilar to the warlock green fire.
Being able to display your guild crest on your shield.
Preferably with 3D texture.
We could have this models for guild shields:
Edit: this could work very well for the Alliance as they are the ones playing medieval Knight’s.
Cheers.
As many as possible as long it wouldn’t end up in the store. 
Star Wars Galaxies Gear system was much better then GW2’s.
It actually makes sence.
You could run your whole shop for a city/location.
You could classically setup a market shop on the streets of tattooine if you wanted it.
If I wanted something, it do be a big overhaul of 2 existing systems.
I was referring strictly to the ability to change the color of your armor, thus having more customization for the same set.
Only one?
Beards for Trolls
Green skins for Trolls (Revantusk tribe)
Green eyes for druids (Emerald dream - Malfurion has them!)
Purple eyes for Draenei (Velen has them!)
Red eyes for Blood Elves (Dark Rangers)
Red eyes for Night Elves (Teldrassil Dark Rangers are a thing, too!)
Ancient Jewelry for Night Elves (matching their hairbands!!!)
This might be enough… for now… 
Long beards for humans, I could use it on my Mage.
Actually I like the basis of your idea but not on a mount. On your character. If you remember the total failure that was Warhammer Online, which got gutted cause of bad management, lying and greed. Well they had this system which allowed you to equip severlat trophies on your character that were class or faction specific. I’d like that if it were possible.