Now that I'm farming old dungeons to get every outfit glamour, It's quite painful trying to 100% each dungeon. As you do more runs you encounter more and more coffers that tell you "Unable to obtain Thingy. You already possess a copy of Thingy." The grind is even worse for High Level dungeons where you're not guaranteed one new item per run like you are in Leveling dungeons, so it's possible to go several runs without getting that last glamour piece you need.

Suggestions I have include putting coffers in old dungeons, or making coffers prioritize a new item over a duplicate.

No one wants another currency, but another idea is that old dungeons could get a token that you can take to an NPC to exchange for gear. So 51-60 dungeons would give a set number of "Heavensward Tokens" which you would take to Rowena's Representative and exchange for dungeon gear that way.