HarderLife

*A lot of these features can be customized with gamerules

Player debuffs when at low health or low hunger points*:

The player will receive some debuffs when being at lower or equal than 4 health points (hp) and lower or equal than 4 hunger points.
Health debuffs: <=4 hp Blindness and Slowness III; <=2 hp Nausea.
Hunger debuffs: <=4 pt  impossibility to jump; <=2 impossibility to break blocks.

Food poisoning*:

Eating raw meat is no longer a valid option, since doing so has a really high chance of inflicting food poisoning to the consumer, which is almost certaintly going to kill you if you don't get rid of it or outheal it rapidly. 

Consumables cooldowns*:

Food, potions and other consumables have a cooldown after being consumed, this varies depending on the type of item consumed, usually better items have longer cooldowns, all these items apply a cooldown only for themselves when being consumed, so food variety can help refill the hunger bar quickly.

Gloom dangers*:

Staying in a dark place (light level of 0) is going to slowly kill you, other than applying lots of debuffs, like blindness and really reduced mining speed, so always keeping a light source with you is necessary when mining or in your base at night time.

Fusion Forges:

The regular Fusion Forge, made from bricks, allows for early game smelting of some materials and for fusion of materials into alloys (like gold-copper made from gold and copper ingots).

If the top part of the Fusion Forge is obstructed with a block while processing items the pressure is going to build up in the forge and it will eventually blow up.*

The Nether Fusion Forge allows to make middle game items, like nethersteel, netherite and ender eyes. (Jei recipes will be implemented soon)

Armor weight*:

Weight is a new effect applied to the player when wearing some types of armor, this effect makes hunger consumption higher when running or jumping and makes movement harder in water, the higher the amplifier the higher the effect's power.

Armor is divided into 3 types: really light armor (leather and chain), light armor (diamond and netherite) and heavy armor (gold, iron and gold-copper).

Very light armor doesn't apply any level of the Weight effect; Light armor applies one level of Weight per armor piece; Heavy armor applies two levels of Weight per armor piece.

Small tweaks:

• Grass paths give speed to the player when walking on them.*
• Stairs, trapdoors, doors and other block variant recipes give result amounts that make more sense based on material quantity required in the process.
• All axes and hoes require one less material to craft.
• Some vanilla uncraftable items have become craftable, like saddles, name tags, chain armor, tridents etc.

Gold-copper alloy set: [currently getting a texture rework]

This new ore is an alloy made from the fusion of gold and copper in the fusion forge. It has a full set of items: armor, tools and weapons, all stronger than iron but weaker than diamond, as a good upgrade before being able to get diamond gear.

Mob powerups & abilities*:

• All the mobs are more resistant to hits, becoming even more resistant when Hard Mode turns on.
• Mob types like zombies and skeletons have a chance to spawn with one or more of this upgrades: better armor, better weapons, status effects (strength or speed), riding an horse (only for skeletons).
• Zombies have the chance to infect the player on hit;
• Spiders spawn cobwebs on the player when attacking (has a cooldown);
• Creepers have the chance of spawning as charged (higher when Hard Mode is on);
• Phantoms have the chance to wither the player on hit;
• Slimes give the "Sticky" effect on hit, which reduces movement speed and jump height;
• Some mobs can apply the stunned effect (more on the specific paragraph), either with 100% chane or a smaller one.

Hard Mode*:

Hard mode will automatically turn on when exiting the nether after the first entry. All the mobs become more resistant to damage when it's on, other than making upgraded mob variants more likely to spawn.

Player spontaneus combustion when in the nether*:

Players spontaneusly combust while in the nether, to negate this effect you need to either have the Fire Resistance effect (inacessible until nether wart is obtained) or the Combustion Protection effect, which potion is possible to make without entering the nether. After having obtained a diamond, some quartz and enough redstone it's also possible to make a red gem, which can be inserted into a gold-copper, diamond or netherite chestplate to grant infinite Combusion Protection without the need to brew potions every time and allowing for longer trips in the nether.

Ghast crying* and exploding*:

Ghasts let out 1-2 ghast tiers every 10 minutes. Ghasts also have the chance to explode on hit, they don't drop anything if this happens.

Creeper spores:

Creepers have spores, when they blow up they release a lingering cloud of spores, if a mob is infected it will be poisoned until the effect ends.

Enderman curse*:

Endermen afflict their target with the ender curse, this effect removes the possibility to break blocks, slows down and slightly reduces the reach of the player.

Freezing effect*:

The freezing effect is applied by ice mobs (Ice zombies [not implemented as of rn] and Strays), this effect causes the player to take freezing damage and be slowed down like what happens when being in powder snow.

Upgraded powder snow*:

Powder snow has been upgraded, while in it the player is really slowed down and the snow becomes harder to break, making saving yourself more difficult.

Stunned effect*:

The mod introduces a stunning mechanic, both the player and some mobs can use it.

The player has a small chance to stun their target on hit when using heavy weapons (axes, hammers, the mace, etc.).

Mobs are divided in two classes: mobs that stun with 100% chance (Goat, Warden, Phantom, etc.) and mobs that have a smaller chance to stun (Ravager, Iron golem, Piglin Brute, etc.).

End crystals shield*:

End crystals have a protective shield, the crystal is immune to any type of damage until the shield is broken. Both naturally spawned end crystals in the end and player placed ones have this shield.

Dropped eggs spawn baby chicken on despawn*:

This mechanic is disabled by default (via gamerule) from the mod version 0.2.0 onwards (not out yet). Since this is still being worked on it's suggested to not turn the gamerule to true, since chickens can spawn in large groups and cause lag.

When dropped eggs despawn, they can spawn a baby chicken, this is determined by how many chickens are around the egg when it despawns, this is made to prevent eccessive lag from too many entities.

New blocks/block variants:

Some blocks have received new variants:

• Bookshelves have slabs, stairs and doors;
• Bedrock has slabs and stairs, unobtainable in survival;
• New stone tiled bricks and deepslate tiled bricks, with corresponding slabs and stairs;
• Reinforced Cobblestone and its variants are just unbreakable versions of the vanilla blocks, used in reworked Jungle Pyramids (or Temples), unobtainable in survival;
• Weaved Cobblestone, causes any entity on top of it to be slowed down and a reduction of jump height.

*Some of these features can be customized with gamerules

Planks and sticks recipes:

Crafting planks and sticks now require a tool to cut down logs and planks, different tools have different amount of results in the recipe.

Tool recipes:

Early game tool recipes, wooden and stone tiers, have string as an additional requirement (either plant string or vanilla string). All axes and hoes require one less material to craft.

Block variant recipes:

Recipes of block variants, like fences, trapdoors, doors, chiseled blocks, etc. have been changed to now require special tools (knives, saws, chisels, etc.) to craft them.

Functional block recipes:

Almost all functional block recipes have been changed, requiring special tools (knives, saws, chisels, etc.) in most cases.

Stonecutter upgrade:

The stonecutter has become harder to craft, but also a requirement to craft some items, like chains and carved wood (important crafting material).

Uncraftable item recipes:

Some vanilla uncraftable items have become craftable, like saddles, name tags, chain armor, etc.

Redstone component recipes:

Almost all redstone components will require a chip to be crafted.

Ore loot tables/smelting results:

Smelting raw ores gives a nugget instead of a whole ingot.

Some ore loot tables have been changed to not drop with 100% probability, such as Coal and Copper.

Some others have been reduced in drop quantity, like redstone and lapis lazuli, or will drop a fragment of the gem instead of the entire thing, like diamonds.

Enchantment removals/changes:

Some enchantments have been removed: Mending, Fire Aspect and Flame.

Other enchantments have received some changes, like reducing max level or general power of the enchantment.

Hardcore hunger*:

This mechanic is disabled by default (via gamerule) from the mod version 0.2.0 onwards (not out yet). 

Hunger is consumed quickly when running or jumping, this is also made worse by wearing heavy armor, which gives the "Weight" effect when equipped, the higher the effect amplifier the more you will consume hunger when running or jumping.

Meat smelting/Loot from burning animals alive:

Meat smelted in a regular furnace will give burned meat as a result, just like what animals will drop on death while being on fire, instead of their respective cooked meat. To correctly cook meat it's necessary to use either a campfire or a smoker.

Enraged cows*:

Vanilla cows have been replaced with new ones, Enraged Cows. This new type of cow will attack and call for help if provoked, they also have a cooldown for being milked.

Vanilla structures:

Some vanilla structures spawning has been completely disabled: Villages and Trial Chambers. Since these structures would allow the player to get some good loot, not only from loottables but also from placed blocks, which are difficult to change in the structure files.

Some structures have been changed: Jungle Pyramids (or temples).

A lot of structure loottables have been altered to give less op loot, especially early overworld ones.

Villager trading*:

The mod completely removes the possibility to trade with villagers, each of them is turned into a nitwit, this was done due to how overpowered trading can be, allowing to skip important steps in progression and ruining the whole purpose of the mod, sadly Mcreator doesn't allow to change vanilla trades so I had to take the drastic measure of completely removing it.

Beds/anchors exploding:

Using beds or anchors in any dimension is never going to make them blow up, defeating players or the ender dragon in a cheap way using [Intentional game design] is no longer a thing. Beds only allow to set your spawn in the overworld, while anchors allow it in every dimension.

• Complete rework of Gold-Copper Alloy armor, weapon and tool sets
• Colored Brick Blocks
• A way to obtain Wither Skeleton Skulls (currently impossible with the mod)
• Complete rework of the Wither
• Jei recipes for the Nether Fusion Forge
• New material variants for chisels, saws etc.
• Rework of all the slab and wall recipes
• Full Ice Zombie implementation (already present in the mod, but incomplete and impossible to find without cheats)
• New anvil that allows armor, weapons and tools repairing without consuming xp, allowing for "Too Expensive" to never appear and stop the process
• A lot of new types of food
• Overhaul of the Ender Dragon fight
• New Deep Dark blocks
• Vanilla biomes changes
• Crucible to recycle tool materials by smelting them
• Full implementation of the Turn Table
• Nether corruption system "infecting" mobs near nether portals in the overworld

A lot more features are planned/in the works, I'm not going to list all of them, mainly because they are not 100% coming to the game, the one listed here are more likely to come but still not surely.