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)
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
Half-Baked Stamina Mechanic
Combat Styles
Dual Wielding
Two-handed
Additions
Door Shield (p l e a s e)
Dagger
Pavise
Riot Shield
Scoped Bow/Scoped Crossbow
Rangefinder
Spyglass + Redstone + Eye of Ender to craft