Well, lets just get straight to the point.
I feel like this game needs costumes that are permanently rare. Meaning that it will not be obtainable ever since the release period of it.
I personally have a lot of cool costumes that are from wayyyyy back when the game kicks off. But they keep coming back and they just feel less and less exclusive. Oddirently speaking, there are probably thousands of costume around and it is getting hard to differentiate between actual good, rare costume.
You may say thats a bit selfish, but i do wana bring meaning to spending tones of money on costume and finally, you stand out beyond all people around. But later just... boom, u can get the costume again in a way easier manner.
It could be something like seasons, 1-2year could be a nice threshold where old costume before that will become exclusive and not obtainable. Especially those old boss costume, event costume and lb award costume. It will only encourage the flow of the game, because the price can increase as time goes on, theres nothing to lose for you. Even cheap costume will gain value later on, and you can proudly present it to show u are an old player and deserve every bits of its glory</