Vanity item called Banner would have a big placement in this game i think
there will be 2 kind of, one of tradeable and other one is just obtainable for the account, never possible to carry to another account. only stashable for other characters of same account.
you can reward them by leaderboard rewards, seasonal event rewards like valentines, winter event etc. and store packets.
tradeable ones must be only country banners. other than i dont think making any other tradeable banners have no mean.