I have suggestion for modifying how [Stone of the Tortoise] works.
Instead of disabling XP gain it would temporarily set your level to XX.
Probably there would be need for set of items, each to set your level to 20, 30, 40 etc.) (or better 22, 32, 40, 50, 56, 65, 75, 85 those are instance levels, right?).
It would make sense to buy that kind of item for max level characters.
It would be usable to go into "retro" raid once in a while instead of creating special XP-denied character.
That way it would be more useful AND would raise sales.
Best regards to all slow XP'ers and retro lovers!