Inspired by Terraria's expert mode and other difficulty-related mods/datapacks, this datapack increases difficulty of Minecraft. After defeating the Ender Dragon, a prompt is given to the player asking to enable expert mode for the entire world. Enabling expert mode makes various changes to all neutral/hostile mobs and to the general way players will play.

Mob Changes:

Most mobs have 2x health and deal 2x damage
Most mobs have 2x loot drops
Most mobs spawn with Regeneration I
Most mobs have 10% knock-back resistance
Bosses have 1.5x health, deal 2x damage, and have additional armor/toughness
Creepers are always charged and have 2/3x fuse
Skeletons and Stray spawn with a Power II bow
Large Slimes and Magma Cubes are larger
Phantoms spawn 3x as large
Spiders and Cave Spiders spawn with Speed I
Zombies have increased reinforcement rate
Creepers give Slowness I when a player is within 8 blocks (disabled by default; configurable in config menu)
Slimes and Magma Cubes have thorns effect
Endermen and Endermites give Blindness I when a player is within 8 blocks (disabled by default; configurable in config menu)
Blaze have thorns effect and Resistance I
Magma Cubes place fire beneath themselves (disabled by default; configurable in config menu)
Zombies and Zombie Villagers give Hunger I when a player is within 1.35 blocks (being hit, more or less) (disabled by default; configurable in config menu)
Silverfish give Mining Fatigue I when a player is within 8 blocks (disabled by default; configurable in config menu)
Spiders place cobweb beneath themselves when a player is within 4 blocks (disabled by default; configurable in config menu)
Spiders now have a chance of dropping fermented spider eyes
Creepers now drop golden apples when killed by an explosion with a 2.5% chance of dropping an enchanted golden apple
Ghasts now have a chance of dropping a fire charge
Silverfish now have a chance of dropping iron nuggets
Witches now have a chance of dropping nether warts
The Ender Dragon now drops a custom item named the Gravity Globe that gives the player levitation while holding it (only drops while in expert mode)
The Wither now drops a custom item named the Lava Charm that gives the player Fire Resistance while holding it (only drops while in expert mode)
The Elder Guardian now drops a custom item named the Sailfish Boots that give the player incremental levels of Speed for every 2 seconds the player runs while wearing them up to Speed IV (only drops while in expert mode)

Friendly Changes:

Iron Golems have 1.5x health and deal much more damage
Snow Golems have 2.5x health
Wolves give Strength I to other wolves within 8 blocks (disabled by default; configurable in config menu)

Other Changes:

Difficulty is changed to "Hard" upon loading pack
Natural Regeneration is disabled
Included is the resource pack for the custom items
Config menu (To access, use the command: /trigger config)
Custom advancements for defeating each of the expert mode bosses

To-Do:

Random chance to replace a mob with a more difficult one
Give players debuffs while in certain biomes
Ability to enable expert mode after killing the Ender Dragon if initially denied prompt
More quality-of-life changes due to the increased difficulty
Method of re-enabling natural regeneration

Any feedback is helpful.

Expert Mode [Datapack]