Ok, so I just had this idea that the Great Sea in Wind Waker may be a flooded hyrule+Termina, why do i say so? well, here the evidence I've found.
Tingle, who comes from Termina, appears at the great sea
The Postal system, who excisted in Termina, but not hyrule, is present at the Great sea
The post sorter Rito dude is a descendant of the postman from Termina
In the Nintendo Gallery, two masks from Termina is behind the figurine maker.
So that's the evidence I have, do you have anything to add? Have you found evidence against or for my theory? Am I completely out of my mind?
Discuss!
1) Tingle appears chronologically in Hyrule in The Minish Cap long before his appearance in Majora's Mask so he isn't exclusively from Termina. Also, there is a Tingle doll in Zelda's room in Skyward Sword, although that's almost certainly just an easter egg and doesn't actually mean anything.
2) A postal system does develop in Hyrule. There is one in Hyrule in (again) The Minish Cap, which comes before both Majora's Mask and The Wind Waker in the timeline.
3) Koboli, the mail-sorter Rito, does resemble the Majora's Mask postman, but this is most likely just to emphasise the relation to postal work. His figurine description in the Nintendo Gallery does mention that three generations of his family have been postmen, but considering the entire Rito tribe rely on the postal service, this is only to be expected and doesn't mean he is descended from the Terminian postman. His figurine goes on to state, "but rumor has it that an ancestor of his in an age gone by was also a postman... ", which seems to be an allusion to the Majora's Mask postman. However, since we know for a fact that the Rito are directly descended from the Zora and that postman was not a Zora, nor lived in Hyrule, it seems likely that this line was only included as a novelty and doesn't actually refer to the Majora's Mask Postman.
4) The masks you refer to in the Nintendo Gallery are simply easter eggs. There are actually 4 in total and three of them are from Ocarina of Time. The Keaton Mask, Bunny Hood and Goron Mask are all on display in the Nintendo Gallery but all three were originally from Ocarina of Time. The other mask is the All-Night Mask, which was from Majora's Mask. These most likely hold no significance whatsoever are are just there as a reference to the N64 games. Again, Skyward Sword has a similar reference to Tingle. Easter Eggs like this are found all throughout the Zelda franchise.
Termina is not just another land that is geographically near to Hyrule. It is some form of parallel dimension or 'mirror world'. It's true nature is still a bit of a mystery but the least we know is that. I don't think The Great Sea is Hyrule and Termina. That doesn't make sense and your evidence to support it is flawed.