The livestream is scheduled for Thursday, March 28, at 2 pm ET (11 am PT). In the description section of the 1m36s trailer, which is titled "Borderlands Teaser Trailer—Mask of Mayhem," Gearbox says, "We aren't done yet!"
There is not a lot to parse from the trailer, though from our vantage point, it looks like an entirely new game. We see a bunch of familiar characters, such as Ellie, Claptrap, and Sir Hammerlock. Collectively, they form a diorama in the shape of a mask.
It's not clear if the characters will be fighting over a mask, or if the mask is a playable item that grants special powers. There are plenty of weapons though, and the entire thing looks pretty cool. We just don't know what the heck it is exactly, in terms of gameplay, setting, and story line.
It is also possible that instead of a Borderlands 3, Gearbox is rebooting the franchise. Pretty much everything is on the table right now, and will be until tomorrow when whatever this is gets announced.