The Legend of Zelda: Breath of the Wild 2 is the 20th game in The Legend of Zelda series, currently in development and set to be released on the Nintendo Switch in the future, in sometime of 2022. It was first announced in E3 2019.
Not a great deal of information is currently known, except that it will be a direct sequel to the critically acclaimed The Legend of Zelda: Breath of the Wild, and is likely to be commemorating the 35th anniversary of the The Legend of Zelda series.
Eiji Aonuma, the director of the Zelda series, had been teasing the next console Zelda game since Breath of the Wild released in 2017. According to Aonuma, the game began as potential DLC for Breath of the Wild, but the team had too many ideas to crank it up into DLC and they eventually wanted to free themselves from the constraint of limiting the potential of new gameplay elements to what was possible in the engine of Breath of the Wild, hence the creation of a new game. 
Aonuma had also mentioned that the sequel would be taking inspiration from Red Dead: Redemption 2 as that game had raised the bar for what could be achieved with large-scale open worlds in video gaming. 
Since the game's announcement in E3 2019 and the release of a brief trailer, not a lot of information has been leaked other than Nintendo's confirmation during the announcement of Hyrule Warriors: Age of Calamity on September 8th of 2020 that they were still in the middle of development on the next game for the mainstream Zelda series. There have been hints however that the game's would establish a more darker tone in its story. 
At E3 2021, it was revealed that the sequel would, in addition to the ground world, feature an area aboveground with floating islands.