There were 5 new (6.2.3) heirloom trinkets added that drop only in mythics, and they drop only for specific loot specializations. The only way to get all of them is to run a mixed group.

Unlikely you can do it with a single team, here is what you need:

Orb of Voidsight - Mage, Balance Druid, Elemental Shaman, Shadow Priest, Warlock
Judgment of the Naaru - Holy Paladin, Restoration Druid, Holy/Disc Priest, Restoration Shaman, Mistweaver monk
Gronntooth War Horn - Retribution Paladins, Arms/Fury Warrior, Frost/Unholy Death Knight
Infallible Tracking Charm - Feral Druid, Hunter, Rogue, Enhancement Shaman, Windwalker Monk
Purified Shard of the Third Moon - Protection Paladin, Protection Warrior, Guardian Druid, Blood Death Knight, Brewmaster Monk

I've only managed 3 so far myself.