How many mounts do you have or have you had? My Hobbit seems to be collecting ponies, and she does not get rid of them when a new one comes along. She has a Bree pony (low-level, slower), a blonde sorrel (first real mount), a chestnut (faction mount that survives more hits), a gray pony (white horse reward for finishing Volume One), and however many seasonal event ponies she has (plus tokens for more). These will soon be replaced with a Nimble Goat, which can be used in Moria.
I could sell a pony every time I get a better one, but they are ponies. My wife might never forgive me, and maybe today I want to ride a different pretty pony. What I would prefer, rather than having an inventory item for every possible mount, is to have a single mount item or power. When I get a new mount, I get a new option for that single mount. Give me a drop-down menu on my character to set my mount choice. (I am aware that I just asked for a pony menu on my paper doll. You cannot have my man card.) If the mount color is not considered PvP-relevant, I should be able to set the appearance to any mount I have unlocked — separate functionality improvements from appearance options, and leave me all my appearance options.
That is a general principle. As with most appearance issues, City of Heroes does it right. You start with a variety of options for what your sword looks like, and when you get a new option, you get a new sword appearance option on your list rather than a new sword to carry around. You unlock costume pieces through badges, task forces, microtransactions, and veteran rewards.
If you have item sets for each class, make that a deed, accomplishment, badge or whatever you call it. Once you finish the set, you permanently unlock that set appearance. You already have a cosmetic equipment tab, don’t you? Have another option on it: pick a completed item set from this drop-down menu.
Or better yet, everyone start using a system like the City of Heroes costume designer. I am looking forward to what Champions delivers. I still need to see The Chronicles of Spellborn’s version, once my addiction to The Lord of the Rings Online™ Volume Two: Mines of Moria™ fades.