I have a small feeling that the next big Zelda game will be to Breath of the Wild what Majora's Mask was to Ocarina of Time. Same engine, maybe many of the same assets, but refocused and retooled into something unique.
Breath of the Wild has one of the best open worlds in any game, ever, and is brought to life with beautiful art direction and sound design. However, anything that would require any sense of structure--story, side quests, characters--usually fall very flat. A game that can take BotW's strengths and iron out its weaknesses would be nearly perfect.
Though who knows how the next game will spring off of BotW. There've been two instances of one game setting a mold, followed by one or two games that deviate from it, and then one game that nearly perfects that mold (Zelda -> Zelda II -> Link to the Past; Ocarina -> Majora -> Wind Waker -> Twilight Princess). So we might get something really out there for the next main Zelda game, and then later we'll get something that returns to Breath of the Wild's mold with more refinement.