I want the classic dungeons back, with some huge caveats. I did not enjoy the Divine Beasts very much, except for Naboris, because while they were nonlinear, they were also so utterly devoid of challenge that there wasn't much beyond the spectacle of climbing around the insides of a big stone war machine. I think Zelda would be better served if it brought back the classic dungeons, with the caveat that things like item gating be removed. Items should be used to make your journey easier, simpler, more convenient, but you shouldn't have to have the Mirror Shield to finish a dungeon if you don't want it. Take the hookshot, for example. It's a great item in concept, but very limited in application most of the time. Now imagine that you can use the hookshot on any surface. You can immediately scale any small structure, and you are granted a huge boost climbing any cliff or wall. It would be a nice convenience that would make exploring the world more fun. Meanwhile, paths can be barred with actual locked doors, and keys shouldn't break, and you have a nice, inventive dungeon that combines the good ideas of the new dungeons with the success of the old.
Bosses, however, are a different note entirely. I like the old Zelda bosses because Zelda has never been challenging, so fighting a visually interesting dynamic puzzle has been good enough for me. However, they can be so, so much more. The first step is to make the bosses about ten times more aggressive. They need to be actively trying to kill you, otherwise it's not a fight. The second step is to make weak points much harder to hit. One of the problems with Breath of the Wild's bosses is that they still have weak points, but if you miss, you still do damage, and the weak points are really easy to hit. They are a crutch, not a saving grace. Think about Smaug from the Hobbit - his armor was nearly impenetrable, except at one tiny, hard to see or hit spot. Now think about the Blights - they have no defense against the army's worth of weapons you can throw at them, and they have a weak point that does critical damage to them. See the difference? Future bosses should be hard to hit, deal a lot of damage, and be constantly trying to throw the player off with rapid volleys of attacks.