Consistency. Or progress if you prefer, but I don't feel there can be progress without consistency. After Ocarina of Time, Zelda has been constantly changing and adding gimmicks, allowing little room for the series to actually grow. Some change is needed to grow, but LoZ has gone too far. Simply put, how can you improve on the horse mechanics when there is no horse? How can the combat from the previous game be improved when the combat has been overhauled twice in a row? How can the dungeons of previous titles be outdone when there's no dungeons left?
Zelda is in the same boat as Paper Mario in that it has completely lost any sense of identity it has ever had. As far as gameplay goes, there's nothing that really separates a Zelda game from the rest. Not anymore. Anything you could commonly attribute across all main titles as an identifier of a Zelda game has been shrinking and shrinking as the series has gone on. Until BOTW, the only two core elements remaining have been dungeons and item progression. And now those are gone. The only thing Zelda has now is its few consistent characters and its setting. And not only are these minor elements of the series, but even these are being cut from their roots with Link becoming less recognizable and Hyrule shifting further from a medieval setting and closer to a sci-fi setting instead.
I'd personally rather the series go back to its roots, whether BOTW be forgotten or branched into a spin off series, but regardless of the decision Aonuma makes going forward, I want him to figure out what he wants the series to be and stick with it. Because as it is, Zelda has just been running around in circles like a headless chicken. After picking a direction, they need to stop with the gimmicks. Stop adding new ideas unless they think they are robust ideas that can stick and make the series better. Don't remove concepts unless they have completely failed and are utterly flawed in concept. And finally, Aonuma needs to stop being afraid to compete with himself. Try to surpass the previous title he made. And if the new game doesn't, learn from his mistakes and try again in the future.