Every MMO has this. When you are active for a certain amount of months or at least every year, you collect an extra currency, or vouchers that give you access to a choice of a unique item that shows how long you have been supporting the game.

Usually it is one voucher, at at the 3 month mark you get to choose between three items, at 6 months you get another voucher, and a new choice of some new items, or you can still pick something from the first set, and the trend continues. It rewards the people over time for continuing to play and support the game and say thanks for all their dedication.

You can do this by giving platinum, or you can give vouchers like I said. All you would need to do is set up another section in the Item Mall for it.