First i don't know what is possible and easy to do. I propose what I have in my head without thinking about difficulty to do it or even if it's possible.

Here are some ideas I had, some may be good, others too complicated, others might break the game or still others might be completely useless, but since I've thought about them, I might as well propose them :p.

By the way, when I tried to place a block next to or above a backpack, it totally disappeared. I don't know if it's just visual or if it disappeared completely. I killed my Minecraft instance in the task manager to avoid saving, maybe. I don't know if it's just me.

Netherite Backpack - +4 Armor (if not resource-consuming and possible). If you are within x blocks of ancient debris, make a sound or something (and maybe a way to know if you're going in the right direction, like a sound level indicator).

Diamond Backpack - +3 Armor (if not resource-consuming and possible). If you are within x blocks of a diamond, make a sound or something (and maybe a way to know if you're going in the right direction).

Gold Backpack - +2 Armor / Piglins don't attack you anymore (same effect as wearing gold armor). Maybe the detection, even if not sure it's really useful; gold farms exist.

Emerald Backpack - Gives off some cool particles, nothing special. Gives Fortune on Emerald blocks. I don't know if it's possible => Villager Trade 5/10/15/20% discounts (Backpack grade). If resources are multi-slot vs. emerald => x% fewer resources, minimum 1. If resources are single-slot vs. emerald => (leather, iron, gold) 1 additional emerald; (diamond, netherite) 2 additional emerald. If resources are multi + 1 emerald vs. resources => x% fewer resources, minimum 1. Something like that. I haven't thought about all the possible trades, perhaps some are completely broken.

Coal Backpack - If connected in block mode to a furnace/blast furnace/smoker (on the left, I think), add one coal every x time to the connected block. Wears Fortune on coal blocks. (Infinite fuel can break the game)

Quartz Backpack - Block mode emits light. Wears Fortune on quartz blocks.

End Backpack - If in the End, grants a speed effect (Maybe a new Backpack or this one, or a new feature => access to Ender chest).

Nether Backpack - If in the Nether, grants a speed effect and Soul Speed.

Sandstone Backpack - In a desert or while walking on sand, chance to passively gather a sand block. Maybe a speed boost too.

Snow Backpack - Wearing it allows you to walk on snow freely. Silk Touch with a shovel on Snow or passively gather snow like the Sandstone one. No fall damage on the snow.

Hay Backpack - Block mode, every x time, x cow/sheep can eat to reproduce themselves (as with wheat). Maybe if it's possible, check the number of cows/sheep around and put a max cap on x block radius.

Melon Backpack - Can drop up to 3 melon slices when harvested. => Didn't test if it's when you pick up the backpack or melon. If Backpack, then Silk Touch on Melon.

Skeleton Backpack - Double bones from skeletons and have a chance to drop additional bones from creatures, passive or not, with bones (Don't know the interaction with Sword enchantment). Additionally, skeletons don't attack you anymore if you don't attack them.

Wolf Backpack - Can tame wolves with an empty slot. Enhanced agility, Jump boost. Pack mentality, depending on the number of tamed wolves around you or following you, accumulating buffs:
1 Leader Pack gives 2 additional health (regenerates after a timer while not fighting).
3 Solid Pack grants Strength 1.
5 Great Pack gives 4 additional health (regenerates after a timer while not fighting).
7 Strong Pack grants Strength buff 2.
10 Dangerous Pack standard creatures such as skeletons, zombies, creepers, spiders don't dare to attack you.

Fox Backpack - Foxes are not afraid of you anymore and will follow you. Enhanced agility, jump boost. Swift sneak bonus maybe or reduce aggro range or when hit by an enemy, trigger invisibility with a cooldown.

Ocelot Backpack - If nearby hostile mobs, grants a speed effect. Fishing is pretty useless anyways, but if you fish with this backpack, count as a rain buff (or another backpack buffing fishing).

Horse Backpack - Can store a Horse (WTF how does it fit) in the backpack. Special key to directly ride it and store it. Or if too difficult, you can ride any rideable creature you cross without a saddle.

Pig Backpack - Acts as a trashcan; everything put in the backpack is eaten by the backpack (maybe with a small delay just in case or a button to delete everything inside).

Sheep Backpack - A weaker version of the blaze one. Immunity to fall damage with a cooldown. Maybe if it's kill you or superior to x damages

Villager Backpack - You can select a villager and he will follow you (or something like that to move villagers easily).

Iron Golem Backpack - 2 ideas: put in block mode, an iron golem bound to the backpack appears to help you; if he dies, there's a cooldown for x time before he respawns (but maybe hard to do). When worn, you can talk to an iron golem and he will drop you x iron ingots, with a cooldown for x time. Or/and additional Fortune on iron blocks.

A new backpack with a totem of undying, banner... giving the hero of the village buff.

Since you suggested sharing ideas with you on the wiki, I'll give it a shot :p.

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