I wish the fierce deity mask in Majora’s Mask had an unlock requirement more difficult that just completion. For how much Majora’s Mask is it’s sidequests, they also repeat every loop which somewhat “erases” them from happening. They still have meaning and impact on the player in both a gameplay and personal sense, and the ending cutscene seems to only imply that they happened in the final “true loop” without the player needing to experience it.
To remedy this, I think the requirement should instead be to complete a sort of golden loop of Majora’s Mask where you do every sidequest in the bomber’s notebook and defeat every dungeon boss to make things as good as they can be for when the day is saved. So that Romani is saved, Mikau’s children are retrieved, snowhead is thawed, and of course Anju and Kafie’s reunion.
I think it would be a beautiful and challenging sequence. The bomber’s notebook would act as a checklist for every person Link needs to help before he saves them. It would also act as a unique final challenge for the time management part of the game as players plan out how which order to complete things in and how to travel there. Majora’s Mask’s heart is it’s side content, and this would be a great way to run through the whole of it one last time for what is currently a superfluous reward which would be more meaningful with this challenge.