2021 same engine, different artstyle, same freedom and new twist.

I would say that it is entirely possible for a Zelda title to release for the holiday season next year. They may release a top-down Zelda game that is a cross platform title: Switch/New 3DS (similar to Fire Emblem Warriors).

Or they could even release Skyward Sword HD, or a HD pack that includes the Wind Waker HD and Twilight Princess HD. These HD versions could be optimised to play better than their Wii U counterparts. The Wii U versions both had frame rate fluctuations at certain parts of the game. The Wind Waker had frame rate issues with boat combat, whenever you were at the reef isles that had boats and cannons, the frames took a hit. Twilight Princess had frame rate issues in the Faron Woods when it needed to render that incredibly foggy area.
These issues could easily be fixed in a Switch version.

I said 2022 for the next main 3d zelda. I'd love to see a 2D zelda in say 2019 as long as they did something new with it and made it really interesting. Also I would pick up an HD Wind Waker in a second so hopefully that comes out too. I think a 2022 Zelda would be perfect. It'd be late in Switch's life, on an install base of many tens of millions, could stick with open world but do some twist on it to give it a whole new feeling from BotW. Maybe its just me but I've always wanted a war zelda game, where the game takes place as ganon is waging his war on hyrule and Link has to turn the tide of the war, infiltrate enemy camps at night, take down dungeon strongholds of ganon's, protect a busy hyrule castle that is under siege, etc.

I'm thinking 2019 for the next 2D Zelda, with SS HD for next year. If we do get a 2D game I hope it'll be a more traditional game (Items, dungeons, no durability or cooking, etc), since that's the kind of Zelda I like the most.

Whenever Nintendo can figure out what they're gonna do for a next console.

It would be 2024 for the next mainline 3d Zelda if the pattern starting with Twilight Princess continues. That pattern has been add an extra year to the wait between the last two installments. The Wind Waker to Twilight Princess was 4 years, TP to SS was 5 years, and SS to BOTW was 6 years. So it would be 7 years to the next if it continues. But hopefully that's not the case and even if so there will obviously be remakes and/or some kind of top down Zelda.

I think they will just do something with all the assets from BotW. I can see a game releasing similar to Majora's Mask, quite similar to its predecessor but still different, if you know what I mean.

2019.

Mainline console game? 2021 and I'm being optimistic (I'm counting that this time they will delay the game just a bit).

Well, for the next 3D Zelda game they can re-use BotW's engine and even its assets, so I think the next game won't take them 5+ years to make. A Majora's Mask-esque sequel with a darker atmosphere would be really cool :p.

Probably somewhere in 2019, but it'll likely be a "lesser" Zelda, in the vein of the handheld games of the past. Maybe 2D/top-down, or something like it - Aonuma has actually stated as much.

http://www.gameinformer.com/b/features/archive/2017/02/14/a-2d-zelda-on-the-switch-is-definitely-a-possibility.aspx

Of course this is not counting a SS HD, which I could see releasing sometime in 2018. Same goes for additional BOTW DLC.

As for a new, big, triple-A Zelda game, I'd say 2020, at the earliest.