The Master Sword got rekt some how, sounds like the Skies and Ground aren't the only parts of Hyrule maybe Underground and Underwater were added as well they may even have a layered open world where although they're not seamlessly connected they're as big as each other. I get the impression they came up with new ideas along the way and rather than cut them and put them in the next game they decided nah lets extend development, I like that I wish more developers could have that option instead of being forced to a deadline.
Most of it are prolly a mix of leftovers and new ideas that came during the first few years of development is my guess.
Well you know what it means .... The climax of the game will incorporate an alien invasion to tease the end of a trilogy ðŸ˜Ž
That'd be way out of place in Zelda!!