I'm not going to predict, but I will say that the latest it could release with me still being ok with such an early announcement is Early 2021 (Jan-April), maybe pushing that into the later part of the first half of the year (May-June), but that's still pushing it. I'm pretty sure Breath of the Wild, Skyward Sword, and Twilight Princess were all about 2.5 years away from release when they got their first trailers. But given that Breath of the Wild's map is already done, the engine is done, most of the models and animations are done ... that seems like too long of a wait.
Also, I could have sworn that there was some pretty obvious evidence that they were making a new Zelda back in 2018, so damning that even it being a unconfirmed didn't really matter, but it seems like I can't find it, and most of the info around is from early 2019 (when Monolith Soft had their hiring spree).
