If they really want to be creative for sales offer things that CANNOT usually be bought, Korean version had the Stat booster pots. Also put pheonix potions back for a limited time only (they are usefull for those times when the raid goes sideways due to lag / crashes etc.. also Zodiac run SUX).
Just putting in vanity items is nice, but in the end worthless to a lot of players.
If its a limited time buy there should be lots of interest.
Maybe put in a LIFETIME purchase option.
Also would be nice to have a GM event or 2, proper events like epic battles or something.