You know what would keep people busy and would be minimal effort and kind of cool. Design a few sick outfits for each expac - ARR, HW, SB and SHB that you can unlock by beating all the new game plus entries for each expansion.

These could be unlocked in the Tataru's Boutique shop thing in Sharlayan or just through an achievement.

You barely have to think anything since it's all there, if it's through Tataru's shop then maybe a few quests about going down memory lane and how recounting the story inspires tataru to design a new outfit and it's not AS monotonous as repeating the same fates over and over plus it adds an incentive to repeat what arguably is the highest quality content in the game, the MSQ.

When 7.0 rolls around, you add one for EW. This would be similar to how the wandering minstrel is inspired to give you new fights after you recount your stories to him.