I'd like to see a new set of mythic weapons and jewelry and rogues should get a new arcane weapon. A new armor set but also add a very difficult quest chain as stated by the op to upgrade. Upgrading again bounds the armor set. When the new set is introduced make the old set unlootable from locked..only their respective elite golden chests. This would satisfy everyone. Demand for new set would be there.
demand for the old elite chests would increase making the older elite content worth farming. And those who spent on a now worthless untradable mythic set wouldn't be upset.