The Wii U version of The Legend of Zelda: Breath of the Wild was playable in the Cemu emulator shortly after it was released, so its mod scene has had plenty of time to get wild. They've added Geralt of Rivia and CJ of San Andreas, turned Link's cycle into a rideable Sonic, made it first-person, added custom dungeons, and got it running at 60fps without the cel-shading.
The Second Wind project is less silly than some of those. It's a mod that its creators call "a large-scale story expansion", which will also bundle in elements of other mods to alter how parts of Breath of the Wild look and play. It's about breathing new life into a game that grew stale for some players—especially after going through one too many shrines that looked the same.
While development is still ongoing (it's apparently 60 percent complete overall, while a planned port to the Switch version is at one percent), according to the project breakdown among the things Second Wind plans to add are eight new areas, including a hub called Ordon Village, 15 sidequests, 38 meals, 28 animals including variants of existing ones, hand-to-hand martial arts, and the masks and gossip stones from previous Zelda games. There's also a shrine overhaul that will tweak all 120 shrines to make them look unique, and give some "new puzzles, alternative solutions, new enemy encounters and more."
Second Wind also aims to incorporate survival elements, crafting blacksmiths, rebalanced mechanics, changes to quivers and unique horses, all of which come from existing mods. And that's not all. There's a Second Wind wiki with more information, and a Discord server with the FAQ.