As title says, I suggest having a special tab in the cash shop for costumes that refreshes daily, cycling between old and new costumes.
However, to make event costumes special, the costumes you buy from the daily tab do not have + (meaning you cannot add bonus to them), they are for transmutation only, and last only for a few days if you do not transmute them.
Also, they can be traded
i hope you like my suggestion, and I hope it happens. There are lots of players who care about cosmetics, including myself.