I have two ideas
1st: On people's birthdays give them a present with something like a VIP voucher? Nothing much but it's once a year! (Maybe have them register there bday)?
2nd: arrows do look cool (the variants you can buy at destiny) but maybe they can have an added effect other then just looks? Would encourage lots of players to buy the new colours! E.g the "God bow" can do 1 damage instead of .5, or the skeleton bow can use more arrows before the metre runs out?