With Hyrule Warriors: Age of Calamity showcasing what world looked like before it was all ruined by Ganon, it was obvious that modders would leverage the new assets to painstakingly "rebuild" the kingdom in the 2017 title.
Now, a modder named banan039 has reconstructed the famous Hyrule Castle using the framework showcased in Age of Calamity, and it's pretty special.
He's rebuilt Breath of the Wild's iconic location, complete with NPCs and items. As you can see from the video, it's a pretty awe-inspiring location; when exploring the ruined version in Breath of the Wild, there was always this lingering feeling that it would have been amazing to see the castle "before the fall", and now we have our chance. According to banan039, Hyrule Town is next.
The obvious catch is that you'll need to run this mod on a PC to experience it first hand, but who knows – perhaps Nintendo will aim to do a similar reconstruction job in a future Zelda title?
Since its not a first party Nintendo title, it might actually go on sale at some point. If you're a big fan of BotW but hate Musou games, its still definitely worth a spin for the novelty factor, just maybe not at full price.
Nice! Must have been a whole lot of work, I'm sure.
As for the question of modding legality (referring to US laws), well, emulation is definitively legal reverse engineering, while it's definitively illegal to pirate games you don't own. ...That's about all we know, really. A lot of this question lies in the Fair Use exception, which is generally vague but essentially comes down to "am I infringing on someone else's profits by doing this". My interpretation is that, in most cases, it should be perfectly fine to rip a game you own, or even to download a game you own if you could achieve the exact same result by ripping (though say, if you own BoTW on Switch, it would be illegal to download the Wii U version). As long as you don't share it, it's fair game...although you probably are infringing if you emulate the game on a device with an official port of said game. As I said, it's kinda vague. And no, pirating GameCube games would still be illegal because, even though Nintendo isn't making any money off of them, they still own the rights to rerelease them at any time, and it can still be seen as infringement if they never port it if, say, you choose to pirate Double Dash instead of buying MK8.