Rather than an "open-world" game, Nintendo has repeatedly referred to BotW as an "open-air" game. The "Breath" in the title probably relates to this open-air concept.
The "Wild" in the title is easier to understand, as it's in reference to the abundance of wilderness that covers Hyrule.
I can't remember exactly where I heard it, but I do vaguely remember that shortly after the title was revealed, Nintendo actually acknowledged that most past Zelda games had the subtitle reference a specific item or person in the game, but with this game they felt like the major selling point was the world so they wanted to evoke the world with the subtitle.
They say a little knowledge is a dangerous thing, but it is not one half so bad as a lot of ignorance. - Sir Terry Pratchett