I don't think that we should really think "Tears of the Kingdom took 6 years re-using assets and world, therefore the next one starting from scratch will take 6-8" because that'll be as misleading as when Tears of the Kingdom was announced and we presumed it'd take less time to make than BotW because they'd already have a foundation.
They decided to go super big with TotK, add literally two new map layers, make a longer story, add new complex physics and pack the map with new stuff and new areas and details to the absolute limit. There was also a pandemic that delayed absolutely everything and that strikes right in the middle of the development, it definitely hurt it badly. And there's the fact that the teams was in no hurry to finish it since Switch already had a Zelda and they could take their time.
Breath of the Wild took awhile to enter full development(January 2013), and Nintendo clearly learned with that since TotK started rather quickly after BotW. I don't know if I'm as optimistic as some that the early development and pre-production started back when TotK was content complete, which would make the next entry 2 years into some kind of development(maybe completed pre-production and one year of full development?), but it could happen, since waiting until spring 2023 to even start pre-production would put the next entry super late into Switch 2's life.
Something that I'm thinking is that while a Hyrule Warriors Imprisoning War telling that story is super more likely, I'd love that my idea for TotK DLC(RIP) where we have a playable Zelda campaign going into the past and everything would turn into a narrative driven linear(or maybe just small areas of Hyrule and open world-ish but not the whole map playable) "spin off". And for Hyrule Warriors we'd get a brand new game in the being of the first with its original story, many characters with unique designs for it and all.
I think a brand new 2D Zelda is fairly likely tbh. Nintendo probably trusts Grezzo to do the work with Aonuma and Fujiybayashi supervising. It'll be 5 years since Link's Awakening (that one took 3.5 years) this year so I think it's finally time, or maybe they'll hold for next year as a major cross gen release.
Nintendo has apparently shown BOTW in 4K and 60FPS at Gamescom for third parties, so next gen patches for Breath of the Wild and Tears of the Kingdom next year are pretty much locks imo, but that'll be more about making sure that TotK stays an evergreen for Switch 2 until its own Zelda than anything.
Wind Waker HD looks perfectly fine, just as good as a cel shading game made for this gen, so just a port (tho 60FPS would be great) would do the trick. Twilight Princess HD definitely doesn't look great tho. It'd benefit a lot from getting a new lighting, touched up geometry and all. It don't need a graphical remake like Metroid Prime Remastered but something like key geometry reworked, all new textures and new lighting(like Luigi's Mansion 2 HD is apparently getting) would make it shine a lot. Just straight ports as a double pack seems more likely tho.
Ocarina of Time 1:1 remake like Metroid Prime Remastered(with QoL changes) or maybe a faithful remake with tweaks like jumping, full camera control and etc, but DEFINITELY not a reimagined thing like FF VII or RE4 is likely the biggest gun that Nintendo can pull and surprise everyone and have a sure system seller, but that'd take an enormous effort that they'd probably just use on the next open air game. But again they did this for a much smaller franchise and sold for budget price so who knows.
- 2024: Twilight Princess Remastered and Wind Waker HD;
- 2025: new 2D Zelda;
- 2026: Hyrule Warriors Imprisoning War or Hyrule Warriors 2;
- 2027/2028: new open air Zelda, I think they'll make travelling through the sea to different islands/kingdoms and underwater exploration the way to have a super packed and giant game instead of trying to make it actually bigger than TotK which seems really unviable to do from scratch.
If the next open air game ends up releasing much later, I think that Ocarina remake becomes more likely, but anyways I'd put remasters of the 3DS remakes somewhere. I think the same effort as Luigi's Mansion 2 HD or Miitopia, having touched up geometry, maybe new lighting, all new textures but maintaining the visual identity and scope.