I really have to wonder how they're going to do the "same Hyrule" thing. Obviously the game is going to have new things to explore, just as the trailer indicated. But with it going to be a direct sequel and seemingly not too far away time-wise from the events of BotW, it leaves the question open of how new structures, dungeons, caves, etc. are going to suddenly pop up across Hyrule when we didn't see them in BotW.
The idea of a huge underground labyrinth that people brought up is certainly a possibility and I believe is decently likely. It'd certainly do it's job of addressing the complaints of BotW's lack of indoor environments and lack of traditional-style dungeon traversing. It'd be awesome to have a huge underground world that essentially works as one huge dungeon made up of smaller, interconnected dungeons that can be accessed from various points in the overworld at the player's choosing. Essentially it'd be the dungeon-version of BotW's open-ended freedom. It spanning across all of Hyrule would allow different sections of it to be vastly different in looks, style, and themes.
With that said, I still have hope that the outside overworld has it's own decently-sized brand new areas to explore. BotW's Hyrule was surrounded on two sides by a giant canyon with lands on the other side-it's certainly possible they can have us leave Hyrule-proper to explore the surrounding lands. Obviously I don't expect a huge expansion of the map assuming the entirety of BotW's Hyrule is back in this game, but having some brand new landscapes to explore is in my eyes essential in a sequel to a game that embraced the sense of discovery.