It's a prequel to Breath of the Wild that will show how our heroes completely beefed it against Ganon. Neat!
So far Link, Zelda, and the four champions are all playable. Here's hoping they bring in non-BotW characters as well for other modes. They haven't announced if there will be other modes, but there have to be, right? They wouldn't forget about Adventure mode, right?
Anyway, this comes out November 20th (!), just in time for you to be almost but not quite finished unlocking everything in Hyrule Warriors.