Early 2021, but honestly it could release as early as Late 2020. It seems like it's quite far along actually. I'm sure they didn't want to give any idea for a date after delaying Animal Crossing and Metroid Prime 4. Early 2021 would put it about 4 years after the first game which would be quite a long time for a game that is reusing assets.

I think I'll go with early 2021, exactly 4 years after BOTW.

Early 2021.
Because the game is reusing Breath of the Wild's engine and assets, that cuts out a good chunk of the development time. Because of that, I think they're already much farther along with this game than compared to how far they were with Breath of the Wild when it was first revealed at E3 2014.
Even though they didn't announce a date, I think that the development team is internally targeting a Holiday 2020 release. But, I expect it to be delayed, because it's 3D Zelda. It always gets delayed. That, and I believe the next 3D Mario game will be the big holiday title for 2020.

There's no way it's coming out in 2020. Based on the fact that they gave no release year, no title, and used the phrase "in development", I think 2021 is the earliest it'll release. I voted for late 2021, but also recognize early 2021 as a decent possibility. Although I would say that about 2022 as well. All the points made about the engine and world existing are valid, but that's why I think it will take about 4 years to make instead of (probably) closer to 6.

Early 2021.

I think they are going to aim for late 2020, but it will get delayed to late 2021. Personally I think this is a better date anyway. Breath of the Wild basically dominated the gaming discussion for the first half of 2017, because it was easily the most significant release at the time. They should do that again with BoTW2. This game deserves to get about a half year of many people's attention. Let it stand out and shine again.

I'd say holiday season 2021.

While they can reuse many assets, the scope of the game is just much greater than Majoras Mask was, They also have not even a name for it yet, so I doubt they could bring it out for 2020, there's just too much to do until then.

It could also release for Goldenweek/Obon Week, but I expect it to be more of the game Nintendo wants to push holiday sales with.

Call me crazy, but I think this could be ready mid 2020. I feel like this has been in development for awhile already and much like MM they are able to use a lot of the BOTW assets. I'd be surprised if it's not out by year end 2020.

Completely in mid 2020s. It will be one of the latest greatest hits of this console like some MGS: PW on PSP. The BOTW was one of the best game of my concole-gaming live so i really waiting fot the next part.

I don’t think it not having a title (which we don’t actually know, I’d say they have an idea themselves at least), is evidence of much. Didn’t Skyward Sword also get it’s title pretty late?

Holiday 2020