I guess someone has probably suggested this before, but I didn't see it in a (very brief, very cursory) search so I'm going to post anyway. Also, if this was available in the Crown store I would be all over it.
I like the idea of costumes, but honestly, I like crafting my own armor and combining them to create unique "looks" -- and I prefer the look I've settled on over everything else.
(If you're curious: Breton light hood, Khajit heavy breastplate, Redguard heavy shoulderss, Redguard light arms and boots, Khajit light pants, Redguard light buckle)
The problem is that as I get closer and closer to v14 I'm going to feel pressure to get certain sets in order to provide specific set bonuses to help me survive some end-game content. As it stands, I probably won't, because I like my character's look waaaaay too much. I'm a shallow gamer that way. When I had a sorcerer I couldn't bring myself to use Daedric armor, even with the 8% magicka bonus, because it covered up my armor!
But costumes are a great workaround for that. You can be wearing all heavy Argonian motiff armor and pop a costume and suddenly you're in Covenant Scout Armor. This even works with Daedric armor, so people who are annoyed by the spiky pauldrons can opt for black-and-tan leather instead.
But wouldn't it be great if you could buy a costume from the store that was essentially a blank slate? When you activate it on your character, it makes a snapshot of the armor you're wearing and converts it into a costume, so that you can activate it and always look like you're wearing that armor. So you can go get that UberDwemerMegaStaminaRegen set (or... whatever) and put it on, and then pop on this costume and it's the look you custom built yourself.
Seriously, I would spend crown points on that one. And since it doesn't actually provide you with any extra skill points, or xp, or damage, or anything other than changing your character appearance, I don't think it would be particularly game-breaking. The only potential issue I see is that costumes are universally accessible between all your accounts, so if you created it for one character it would be available for all the others. Honestly I would be OK with that too.