I'll start off with some of my personal wish list items:
Wound Dressing/Anti-Septic Soaked Dressing
I've seen this one requested before. This is a popular roleplay item since many of our characters tend to get injured on the regular. Has been present in the game since Vanilla. Perhaps they could be crafted via high level First Aid giving players more of an incentive to level that profession.
Gina Lang's Customized Spellbinder Robe
I LOVE this robe, it's so cute.

I know it's just a cropped Spellbinder Robe but there's just something about it that made me always want it. I feel like it would be a fairly simple addition. My idea was for it to be either a BoE Green item or a level 20ish quest reward both factions could obtain.
Additional Swashbuckler's Shirts
Swashbuckler's Shirts are very popular for casual outfits and having a wider choice of colors would be wonderful. I know there exists blue, green and purple versions that are currently exclusive to NPCs. My personal favorite is the one worn by Martie Jainrose in Lakeshire. Like their black, white and red counterparts these would be crafted through tailoring.
Darkmaster Gandling's Helm
I know that Ironweave Cowl uses the white version of this model which is very nice but I really would like to see this version too. Perfect for a Necromancer transmog. My idea was to have the item drop from Gandling himself, or perhaps an Argent Dawn quest reward involving Scholomance.
Tier 2 Recolors
These were added in TBC alongside recolors for many Tier 1 and Dungeon sets. The nice thing about these is that they're all class, meaning players can use these sets without restriction while the original Tier 2 remains untouched. They don't even have to be the same colors that were introduced in TBC I would be interested in seeing NPC exclusive colors or even completely custom colors! I could see these being obtained at level 60 either through quest rewards or have them added as drops in the custom endgame dungeons.
That's all I've got for now.
I look forward to seeing your suggestions as well!