All sorted? Then we'll continue...
The specific area that Aonuma had hoped would make it into BOTW was an underground cave system — something that the developers confirm is available in Tears of the Kingdom.
The game's technical director, Takuhiro Dohta, stated that the team had originally decided that flying would not be an option in BOTW, deeming it too much for the limitations of the Wii U. At the risk of losing this mechanic, Aonuma stated that if flying wasn't available then he believed that Link should be able to go underground instead. The team settled on the friendlier option for Wii U development, and added a flying mechanic instead of attempting to create an entirely new layer for the world:
Dohta: Actually, the previous title, The Legend of Zelda: Breath of the Wild, was originally developed for Wii U, so there were restrictions in development. There were a lot of ideas we wanted to implement during its development, but we made clear decisions on what we wouldn’t do in that game. For example, we decided that it wouldn't involve flying. Then Aonuma-san kept saying, "If flying is out of the question, I want to dig underground!". And we'd respond, "Oh no! Please don't make us develop that too!".
Dohta goes on to explain that TOTK's development began with the team compiling all of the things that they had wanted to do in the previous game. Following Aonuma's wishes for BOTW, this included the implementation of caves.
Tears of the Kingdom's art director, Satoru Takizawa, goes into a little more detail about what we can expect from the cave system in the upcoming title, and it looks like the focus is once again all about encouraging exploration:
I think that players who find a cliffside cave in this game might start wondering, "Is there one in the cliffs over there too?". When a place – even one that's already familiar – is augmented with something of worth, you begin to see the world in a different light. Even as the game's developers, we started seeing the landscape in a different way while working on the game. I believe the way players explore Hyrule will change too.
This third part of the Ask the Developer interview goes on to discuss a range of topics (besides caves) including how Tears can be considered a return to "traditional Zelda", the implementation of the sky islands, and how the game's music needed to be adapted for these new layers. You can check out the full interview over on the Nintendo site.