In the trailer, we see mystical forces circulating around what appears to be the rooting manifestation of darkness. Deep in some temple- like ruins, we see Link and Zelda as they explore the area. They find the rooting evil. Hypercutting through several intense moments, the camera stops to show a zombified body spring to life. Cut to the outside world, where Hyrule Castle lies in the distance. a rumbling occurs as the castle starts to rise.
It ends with one sentence.
"The sequel to The Legend of Zelda: Breath of the Wild is now in development"
Breath of the Wild was a massive success when it released on Nintendo Switch. The open world style paired with the temporary usage of weapons made for an entirely new type of Zelda game. We can only assume that whatever this game is, it will follow a similar mechanic theme as before.
There is no title, no release window, no further details. We imagine that this was announced pretty early on. So it will probably be a good while before we learn more, and even longer before it releases. Stay tuned.