I really like the addition of the Outfit system (though I still disagree with Outfit tokens being so expensive and bought per character rather than per account) and think it has been a great addition to the game, and yet I think it could still be made better. I'd like to give some suggestions that ZOS will hopefully see and leave this open for others to add onto.
Costumes conflict with the Outfit System, and most costumes have pieces of gear or clothing that are exclusive to those costumes: I think it'd be great to break up pieces of the costumes (shirt, pants, boots) that players already own and add them to the Outfit system to make outfits out of, just like what's already done with motifs the player knows on their characters.
As an example, I may love a shirt or an accessory on a shirt piece on the pirate costumes, and yet I don't like the boots and the pants. If this were to be implemented, I could put on my own choice of pants, boots, gloves, you name it, and then the possibility of outfits would be even greater.
We already have the option to hide helmets in the Outfit system, but we don't have one to hide other slots. It may sound silly to have the option to hide other parts of gear, but I've heard some people talking about it, especially when it comes to hiding shoulders.
I've also seen bags and pouches and even backpacks on different motifs and costumes, and I think it'd be cool to add a whole different slot to be able to add those to our outfits, despite it maybe mismatching, looking a bit off, or making horrible clipping. A big, big suggestion I've seen among players it the option to be able to wear these backpacks outside of the Treasure Hunter costume, or Breton robes, etc.