This was prompted by the Guardians Armor Pack DLC sale. Promotional armors look awesome, but sadly are restricted to only costumes. This is great for keeping the game P2W free (mostly), but sometimes you just want to wear those armors as armors. So, what I'm proposing is to make these items either exact clones or slight variants of existing armors, but have them upgrade as you progress through the game.
For example, let's take the Celestial Set. If you bought it as a Recruit, it would have the same stats as proto armor. When you became Apprentice it would have the same stats as Breaker armor, but the amount of elemental defense is converted to more normal and it resists stun instead of fire, shock or freeze. From 3* onward it would follow the path of Angelic -> Seraphic -> Valkyrie, but would have stun resist instead of poison and freeze vulnerability instead of fire.
This way you can buy a promo armor at any point and actually use it as real armor, but not have the problem of buying T3 gear in T1 or the Honor Gear/Starter Pack problem of having useless T1 or T2 gear as a Vanguard. And probably more people will buy the promos for the chance of getting actual useful items.
Ideally these armors would just have been released as permanent content armors with these stats instead of as promos, but I know that's not going to happen. :p
i also wish they were armors that have a varied stats that differs a little from the original armors. Alas it is not so, but your suggestion would mean that when ever promo cosmetic item is invented, there will be a copy that has a small difference in visual design but will be permanent content to the game. I don't think it's a bad idea, but at the same time, the game would go pay 2 win all the way sense most of these armors can only be obtained via cash money up front. so this is 50-/50+