I remember reading elsewhere (maybe old Nintendo Power magazines) that originally the game was planned for the 64DD and would use the expanded memory for a full week. When the 64DD failed and the game moved to a regular cart they had to scale it back to 3 days because of memory limitations (and even then the game still needed the expansion pak so the game could properly keep track of all the scheduled events without running into a memory error, because unlike Ocarina before it the scripted events in Majora's Mask will continue to happen regardless if the player is there to witness them).

I also recall old screenshots showing there was to be an Adult Link mask but the devs dropped it for being ultimately unnecessary as, unlike Ocarina, Link's equipment isn't limited to only being available to a specific age. They instead repurposed it into the Fierce Deity's Mask as a 100% completion bonus (and a necessary requirement to see the full ending).

SkullKid rolls up in Termia like