I think it's about time the costume system is overhauled to better integrate a lot of the more armour centric costumes into the outfit system.
There have been numerous costume releases that feature armour pieces that are simply too good to keep excluded from the outfit system proper, that seem like outfit motifs that are restrictively packaged as a single "wear all" item within the costume they are a part of. Examples that immediately spring to mind are the helmets of the Pact Mother Elite Ensemble or Grim Harvester costumes, or the Arena Gladiator shoulder and bracers components. Many of the costumes are quite evidently designed as outfit styles given each part can seemingly be divided into its respective slots as regular motifs are when looking at them on your character. So it would be great if the costumes that fit this format can be divided and utilised in the outfit station with the base game and DLC/event motifs for even greater customisation and more variety in appearance options.
Obviously there are many costumes that do not fall into this format such as dresses and other thematic outfits such as the logger or barkeep. But these fit within the theme of a costume, rather than a set of armour like the costumes mentioned earlier in the post are. If ZOS could add this feature and convert many of the armour set costumes into outfit styles I think that would be ace.