The Monster Multiverse

Additions

Method of restarting the ender dragon boss fight
Infinite generation of islands in the end, populated by chorus trees, end city structures and ships, and shulker mob
Dual wielding
Attack strength mechanic
Attribute system
Shield, spectral arrow, 14 types of tipped arrow and a lingering potion for each type of potion.
Elytra for Gliding
Subtitles
Off-hand slot, and dominant hand option
Igloo structures that generate in cold biomes
Skeleton trap horses that spawn during thunderstorms.
13 new blocks: grass path, frosted ice, 2 new types of command block, chorus flower and plant, end gateway block, end rod, end stone brick, purpur block, pillar, slab and stairs.
New items: beetroot with seeds and soup, end crystal, normal and popped chorus fruit, luck potion and dragon's breath
5 new splashes.
4 new particles:
sweepAttack, dragonBreath, endRod and damageIndicator
4 new status effects:
Levitation, Glowing, Luck and Bad Luck.
2 new enchantments:
Frost Walker and Mending
5 new statistics:
sleepInBed, sneakTime, pickup, drop and aviateOneCm
Lots of new NBT tags
3 new gamerules
disableElytraMovementCheck, spectatorsGenerateChunks, spawnRadius
150+ new sound events
Support for Chinese, Japanese and Korean, and probably other IME entered languages

Changes

Changed the layout of the end island pillars and the ender dragon boss fight.
Skeleton AI is more difficult.
Sword blocking removed, and several other weapon techniques added.
Tool damage is rebalanced.
Sharpness and protection enchantments are nerfed.
Status effect icons are now visible on the main screen.
Health regeneration is rebalanced.
Fishing is rebalanced.
Brewing stands now require blaze powder.
Enchanted golden apples are no longer craftable.
Maps changed.
Loot found in chests generated in the world is rebalanced.
Death in hardcore mode allows the player to continue in spectator mode.
Boats are improved.
Stronghold generation increased to a maximum of 128 strongholds.
Large oak trees generate in forests again.
Increased fire spread while raining.
Glass panes and iron bars have a thin appearance when not connected, as opposed to a crosslike appearance.
Mobs and players now push one another around.
Death message is now displayed on the death screen.
Mob and player heights are slightly modified.
Resource packs can specify models depending on damage value, item state, and hand.
Structure data now comes from /assets/structures.
Loot from mobs, fishing and chests is now determined by loot tables.

Performance and optimizations

Particles are no longer entities
Block ticking
Mob pathfinding
Memory usage when connected to servers