Since exotics arn't rare at all, there's needs to be one more tier higher than exotic. Thus Relic items would become the hard to get once in a blue moon you'll see it on a person. These relic pieces could be class specific and requires a bounty quest line similar to the exotic quest lines but obviously more challenging and more difficult to complete. These challenges would need to be nearly impossible to obtain, similar to the legendaries in WoW back in the day, where you had to literally have to farm one boss for months at a chance to get one piece of 30. Or just to make it boring lazy and stupid, you can make it a random world drop that literally has a 1 in 9999999999999 chances of it dropping.