2 extra dungeons, 1 on the moon before you fight Majora, or perhaps the clock tower itself. And... I don't know just another one.
More uses for some of the masks.
A time trial for rematching the bosses.
Have the item storage, why was there a stall for it but there was never someone there?
Hard save at states, quick save anywhere.
Tatl, Character Bios.
Of cource improve graphics.
Better songs, asthetics ect.
Smoother movement.
A battle dungeon.
Some extra stuff to buy... tunics or something...
Better wallet upgrades 100, 200, 500 screw that... at least
TP's wallet system.
Some extra secrets.
Really enhance some of the themes, make them as recognisable as
Oot's