In various interviews around and after E3, series producer Eiji Aonuma has confirmed that the development of the sequel to Breath of the Wild ("Breath of the Wild 2") started as DLC ideas, where he wants to revisit that Hyrule again. And judging from the E3 trailer it seems like they are going to place this game in the same Hyrule again, but with an underground world to explore instead of the Sheikah Shrines and Towers.
Of course it is still too early to tell, since the game could offer so much more, but for the sake of this article we will assume that Breath of the Wild 2 mainly will be an expansion of the world of Breath of the Wild, where you explore Hyrule's underground.
This would be a risky move, because Breath of the Wild owes a huge part of its success to its completely open and wondrous overworld, which you can explore as you please and without limits (the Great Plateau tutorial section aside). You climb and glide everywhere and this makes the game so fun that it's a must buy for every Switch owner.
And the success shows: it's the best selling Zelda game of all time with over 14 million copies sold by end of March 2019 - probably some more by now. It's the first Zelda game to break the 10 million mark and it keeps selling.
But... a sequel will have a hard time replicating this success if it re-uses the same world to do something different with it. Even if this expansion will be massive, it won't be able to carry the same sense of discovery and wonder that Breath of the Wild first had. Most people, who bought the first game, still won't have done everything there is already – they are probably still in the middle of exploring, searching for Shrines, hunting Koroks and so on. Or they were exhausted by it and stopped playing.
The main selling point of Breath of the Wild was its world and that's right there on the package. To many people the sequel won't have any real value if it just comes with the same world again. Why buy something that you already have? For some new caves?
A game that takes the same world, but changes it in many ways, will mostly be interesting to the people who have completed Breath of the Wild and can't get enough of it. But even then, getting a completely new world would probably be more exciting than re-exploring the same world to discover the differences and a new underground world below it.
In addition, Breath of the Wild looks best when you're out in the open. The engine of the game really was made for beautiful, open landscapes. It wasn't, however, exactly made for caves and dungeons, where for example the underground sections of Hyrule Castle can look somewhat dull...
So, here a lot of work would be necessary to make the underground sections look appealing, but it still won't have the same appeal as a beautiful new landscape. As already said, the main selling point of Breath of the Wild is on its package and the underground cave expansion sequel won't have that... It will have dark caves instead.
That being said, it still could be an interesting idea to add an entire massive underground world to the Hyrule as we know it from Breath of the Wild. It could be one massive maze, where you make progress by finding different entrances on the overworld. It could be similar to the Skeleton Forest & Skull Woods from A Link to the Past, but on the scale of the entire Breath of the Wild game world. This would allow for a much more classic, maybe even "Metroidvania"-style progress, while maintaining the freedom in the overworld of the predecessor.
At the same time it's also possible that they will expand the overworld itself. Instead of being stopped by an invisible wall in the middle of the ocean or the desert, you could now travel even further and explore new lands beyond the familiar Hyrule of Breath of the Wild... Maybe you can even use the underground to travel to entirely new places.
This is all yet unknown and there are lots of possibilities to make the sequel very amazing, but from what Aonuma has given us so far, there are reasons to be concerned if the sequel can really live up to the success of Breath of the Wild under these conditions.
