As with all suggestions, exact function and numbers are simply for reference and not required for the suggestion to be added, if even added at all. This list is stuffed full of ideas, most of which I wouldn't ever expect to be added, I'll try to add additional suggestions to the list to make sure it all stays in one spot.

Mechanics

• Visually hide offhand shields when not blocking or holding weapon
• Prevent tools/weapons from interacting with grass/flowers/bushes/etc. except for hoes
• Hoes deal increased damage, slightly longer reach, and have innate Sweeping Edge IV, even while moving
• Knock back arrows/thrown potions similar to fireballs (good luck hitting something that small though)
• Prevent damage to mounted entity (boat/horse/minecart/etc.)
• Gold armor has protection against magic damage (toErlpNid)
• Tridents and pickaxes ignore armor (toErlpNid)
• All armor except leather and chainmail armor decrease movement speed
• Invisibility potion effect hides armor, but spawns some extra particles
• Invisibility is removed when attacking, being attacked, or bumping into a mob
• Riptide functions in air at half power (full power in water or rain)
• Infinity enchantment works without arrows
• Infinity can exist with Mending
• Health bars for mobs
• Damage numbers on hit
• Timer overlay on potion effect icons
• Trinket mod: spyglass can be equipped in the head slot to allow zooming-in with a hotkey
• Trinket mod: shields can be placed in back slot to automatically block damage from behind

Half-Baked Stamina Mechanic

• Stamina bar in place of hunger
• • 2x more stamina points than vanilla hunger points
• • Regenerates over time
• • • Sitting or crouching in place: +8 stamina per second
• • • Standing still: +4 stamina per second
• • • Walking: +1 stamina per second
• • • Moving slowly (blocking): +2 stamina per second
• • • Running: -2 stamina per second
• • • Walking with slowness effect: -1 stamina per second, +1 for every additional tier of slowness
• • • Running with slowness effect: -3 stamina per second, +1 for every additional tier of slowness
• • • Fast swimming: -4 stamina per second
• • • Jumping: -12 stamina
• • • Attacking: -8 stamina
• • • Blocking: -24 stamina per hit
• • Instantly restore some health and stamina with food
• • • Hunger value for stamina
• • • Saturation value (divided by 3) for health, with a minimum of 1 point (half a shank)
• • • Stamina used to heal naturally
• • Natural healing depletes 16 stamina for every half heart
• • 0x heal rate with no stamina 1.0x at half stamina, 1.5x at full stamina

Combat Styles

• Dual Wielding

• • Unable to block explosions or projectiles
• • No manual blocking
• • Automatic parry of melee attacks if using a sword or dagger in offhand slot (deals 75% of offhand weapon damage to attacker instead of taking damage)
• • • Cannot parry Ravager
• • Parry cooldown depends on offhand weapon attack speed (1.0x = 2 sec, 2.0x = 1 sec, etc.)
• • Does not prevent use of main hand
• • 80 degree protection arc (compared to shield's 100 degree arc)

• Two-handed

• • Deals 1.5x sword/axe damage when offhand slot is empty
• • Attack speed is 30% slower
• • Only affects swords and axes
• • Manual blocking in a 100 degree arc
• • Reduces knockback by 50%
• • Cannot block projectiles or explosions
• • Jump attacks can disable shields

Additions

• Door Shield (p l e a s e)

• • No instant-block
• • • 0.5s "wind-up"
• • Cannot be disabled by axe
• • Slowness II while blocking
• • 120 degree protection arc
• • Blocks all damage and halves knockback
• • Can be dual-wielded for a 180 degree protection arc
• • • Blocks all damage and knockback
• • • Covers most of your FOV, but you can kind of see through the windows at the top
• • • Main hand can attack/bash for 6 damage at 0.7 attack speed
      Inspired by https://www.youtube.com/watch?v=70Z2t7nq_sU

• Dagger

• • Sword damage minus 1
• • 2.0 attack speed
• • Cuts through webs
• • Shorter attack range
• • Blocking and sneaking with no offhand for 3s turns player invisible until they attack or stand
• • Deals 2x damage while invisible (because backstab mechanics in minecraft is stupid)

• Pavise

• • 100 degree protection arc
• • Shield recipe, but with diamond instead of iron
• • 504 durability
• • Can attack/bash while blocking with an attack stat of 4
• • 2s cooldown between bashes
• • Functions as an axe in dual-wield mode
• • Can be disabled by an axe as normal
• • Provides 1 armor point while held in offhand

• Riot Shield

• • Transparent aside from frame and handle
• • Shield recipe but with diamondglass instead of wood
• • • Create diamondglass in smithing table by combining diamond and glass. Surprising, I know.
• • 672 durability
• • Instant block
• • Covers most of your screen, but less than Door Shield
• • 100 degree protection arc
• • Melee attacks disable shield for [0.5s * dmg]
• • Arrows stick harmlessly into shield, thrown potions are neutralized (thrown water potion), and fireballs are automatically deflected
• • Can sprint while blocking
• • Initiating block does not cancel sprint
• • Blocked projectiles do not cancel sprint
• • Blocked melee attacks cancel sprint, reduced to 0.8x damage, and disable shield for (0.25 * (dmg + 1)) seconds
• • Blocked explosions cancel sprint, have 0.5x knockback, and are reduced to 0.5x damage

• Scoped Bow/Scoped Crossbow

• • Combine spyglass with Bow/Crossbow in smithing table for Scoped Bow/Scoped Crossbow
• • • Hold crouch to zoom in
• • • Spyglass Zoom mod: scroll wheel adjusts magnification while scoped
• • Spyglass can be removed with a grind wheel (Spyglass moves to right-hand slot, bow/crossbow remain in top-left)

• Rangefinder

• Spyglass + Redstone + Eye of Ender to craft

• • Combine Rangefinder with Bow/Crossbow in smithing table for Advanced Scoped Bow/Advanced Scoped Crossbow
• • • Hold crouch to zoom in
• • • Shows arrow trajectory while scoped
• • • Spyglass Zoom mod: scroll wheel adjusts magnification while scoped
• • Rangefinder can be removed with a grind wheel (Rangefinder moves to right-hand slot, bow/crossbow remain in top-left)