As someone else pointed out, most people would still use an exotic piece of armour and give up any bonus from a full set if it isn't that great a bonus. But they could do it so equipping a full set of armour activates an exotic perk. Maybe have each piece of armour have its own exotic perk, but you can only activate one at a time. Edit - For example, the Hunter IB gear. The helmet could have celestial nighthawk gg perk. Arms could have Don't touch me invisibility perk. Chest could have alpha lupi quick revive perk. And legs could have bones of eao extra jump perk. On each piece there is an extra activation node on the perk tree, once you equip the full set that node is usable but you can only activate one perk out of the 4.