Allowed for new costume pieces
New and interesting item systems could be designed to fit into a cash shop scenario. This could also make for an interesting character customization system. I for one hate looking exactly like everyone else in an MMO; you lose a sense of identity and it quickly deteriorates my sense of immersion. If a games cash shop allowed for new costume pieces or facial customization options, I feel that this would be a great way to increase revenue, while not breaking Phoenix Dynasty Money in any way.