Let's take a closer look at the Equipment and Items menu screens again, because speculation is always fun. We haven't seen the Collection screen yet, so an important part of the picture is missing, but it seems like each screen has a clear purpose. "Items" is for what you can put onto the ZR button for active usage. You can also swap these by holding down R for a quick menu, just like in all the Zelda games made for Nintendo Switch.
The "Equipment" screen on the other hand includes everything else that can be changed somehow. Like in the Nintendo 64 original, this is where you alter between the three different swords, shields and tunics. But it now also comes with the "toggle items". These are different from the "action items" on the ZR button, because all they did was to turn a piece of Link's equipment on and off. This includes the Lens of Truth, which are now a pair of goggles, as well as the masks. And in Ocarina of Time 3D both the Iron and Hover Boots became a similar item.
Since all of these used to clog item slots in the past and there is now only one item slot, they have moved them onto the D-pad for convenience. But since the D-pad also triggers the ocarina and Navi, they had to come up with a solution here, where pressing up is for changing headwear and pressing down is for changing footwear. It's not exactly clear yet how you will quickly swap between the two alternate boots, or between the goggles and your mask, because going into the menu for this would be inconvenient. My guess so far was that you will hold down the button for that.
Anyway, there is one more item slot below these four. It's probably for the ocarina, since this item also gets activated via the D-pad. And everything that is completely passive has moved into the Collection screen, like the Goron's Bracle, Silver and Golden Scale and Gauntlets, the Bomb Bag, and so on.
It could also be the Stone / Shard of Agony, which you can simply turn on and off here, similar to the Seashell Sensor in the Link's Awakening remake. But I personally hope that this item won't return at all. It was originally a gimmick meant to sell the Nintendo 64 Rumble Pack, as a means to find all the secret grottos. And these grottos were so obscurely hidden at times that it's basically a remnant of the NES era.
Ideally, in the remake you will have clear visual clues on the ground whenever you can find a grotto. They can be subtle, but they should be there. And these clues should even make it also clear whether you have to use bombs or the Song of Storms to open it. Though, there are different clips on the Japanese website and in the trailer, where young Link fights the Wolfos at the entrance to the Sacred Forest Meadow. And there doesn't seem to be anything there that indicates a hidden secret, but it's hard to tell and they also could have moved the location slightly.
Alternatively, they could just give the new "Goggles of Truth" the ability to easily spot hidden grottos. This would probably be the most streamlined solution – combine both of the secret-finding items into one.
Let's move on to the "Items" screen, because that's where the real question marks await us. It got split into three different sections. One is for young Link's items in the top right, one is for adult Link's items in the top left, and everything in the bottom half can be used by both. Each slot has a shape that resembles the item somewhat, so you can make educated guesses what goes into what.
For young Link you get the Deku Sticks, the Fairy Slingshot, the Magic Beans, and the Boomerang. It's curious that the Magic Beans seemingly remain as an action item in the new system. Maybe they are functioning differently to justify this, where using them anywhere might lead you to the nearest spot of soft soil. But if they work exactly like in the original, then this will be just an unnecessary waste of space, because a simple prompt at the soft soils would have done the trick.
Adult Link gets the Hookshot, Fairy Bow, the three magic arrows, and the Megaton Hammer. Not necessarily in the exact order, because here it's a bit harder to tell what goes into what slot. You'd expect the largest slot for the hammer, but that one is clearly shaped like the bow. But it doesn't matter, because everything is accounted for.
Things get more interesting in the shared section. First you have Bombs and Bombchus. Then you have the Deku Nuts below, an open slot next to them in the same hexagonal shape, and then three magic spells. Then comes a square slot and an octagonal slot below it in the bottommost row. That row has otherwise slots for the Empty Bottles, five in total, which means that they have added a new one.
Either the square or the octagonal slot in the bottom right could be used for the trading quest items, given that they haven't made those prompt-based now. But there are actually two different lines of these, one for the child and one for the adult era, but they should combine these into one, because there is no good reason not to. In the original game Zelda's Letter was replaced by the masks, which now function differently. They could just remove it from your items and then later use the same slot for the Pocket Egg, which eventually ends up as the Claim Check. It would also explain why this slot sits in the shared section.
Now, this still leaves two items unaccounted for. One idea would be that they are introducing the Pictograph Box from Majora's Mask already. They love giving you the option to do selfies in the 3D Zelda games, and Nintendo loves dedicated photo modes in general. They may even introduce a new side quest line where you have to take photos of monsters and other things, but it doesn't have to go full "Hyrule Compendium".
Then there is the open slot next to the Deku Nuts. The identical shape suggests that this might become a similar item, so potentially another consumable, maybe also something that can be thrown at enemies. Maybe it's something like the Scent Seeds from the Oracle games, which lets you attract and distract enemies.
Or, if they were to redesign the Ice Cavern to turn it more into a proper dungeon, then maybe the Blue Fire could evolve from a bottled item. Maybe you have something like fire crystals that can be thrown and turn into a flame on impact. This could even give you more combat options for burning enemies, in addition to Din's Fire and the Fire Arrows, or the possibility to light fires even when you're out of magic. But that's just an idea. It would be totally fine if the Blue Fire worked the same as before, especially when there is one more bottle to find.
In either case, if there is a new consumable, it could mean that you might be able to expand its totals, like with the Deku Nuts and Sticks before. Well, it could get the same treatment as the Bombchus, where their total is maxed out from the start and you're not using them as frequently anyway. But if it's another plant-based item, and maybe something that appears early in the game, e.g. "Deku Fruits", this could warrant a new reaction at the Forest Stage.
