I'm not sure how possible this is for modders, but for awhile now I was inspired by the "Extra Hard Mode" Bukkit plugin to make my own idea for a mod that changes pre-existing features to make survival Hard mode more difficult. In order to make it stand out, I tried coming up with some changes that are more unique/interesting than just making all mobs have more health/damage. Granted, that doesn't mean there aren't some buffs to mobs, they're just not quite as intense as other mods that try to increase the game's difficulty. You can see the list of changes I've come up with here:
Minecraft Delta Hard/Deltacore Mod (1.12)
Block/Item Changes:
-Bonemeal now only boosts a crop by one stage per usage.
-Beds will no longer skip the night, but will still set your spawn-point.
-Diamond Pickaxes can no longer be used to obtain Obsidian blocks (Elaborated on more in Overworld Changes).
-In order to prevent the lava-water Nether Portal trick, buckets will no longer be able to pick up lave source blocks.
-Tree saplings can no longer be obtained from leaf blocks (Meant to prevent tree farming).
-Seeds can no longer be obtained from grass, making it more difficult to obtain crops early.
Overworld Changes:
-All ores except Emerald and Diamond now have smaller veins.
-Silverfish block veins will now naturally spawn underground in any biome, in addition to spawning more frequently (Spawns at the same layers and frequency as iron ore).
-Obsidian blocks can now instead be found in 1-2 stacks within Dungeon (21.8%), Mineshaft (16.9%), Desert (23.5%) and Jungle Temple (15.3%) chests.
-Obsidian blocks in Village Blacksmith chests are now more rare (25.6% -> 16.2%).
-Dungeons will spawn more frequently throughout the Overworld.
-Dungeon spawners have now been reworked to include additional mob spawners:
Zombies (20%)
Skeletons (20%)
Spiders (15%)
Husks (10%)
Strays (10%)
Creepers (2%)
Slimes (8%)
Zombie Pigmen (10%)
Silverfish (5%)
Property Changes:
-The difficulty setting will now be locked by default, with the name of the difficulty being “Delta Hard” (When playing in Survival) or “Deltacore” (When playing on Hardcore Mode).
-Dirt, cobblestone (Not counting mossy cobblestone), wooden stair, and wooden plank blocks are now all affected by gravity similar to Sand blocks. In addition, the slab variants for wooden plank and cobblestone blocks will also be affected by gravity (Meant to make building a shelter and spelunking more difficult).
-Fire spreads much more quickly now.
-Lighting bolts will now create a fire explosion when striking the ground (Explosion Power = 2).
-Golden Carrots and Glistening Melons now require Gold Ingots in their crafting recipes instead of Gold Nuggets.
Mob Changes: Passive:
-Domestic mobs that produce food will now spawn much less frequently throughout the Overworld.
-More expensive items are now necessary when breeding specific mobs:
Pigs & Rabbits - Golden carrots
Cows & Sheep - Golden apples
Chickens - Glistening melon slices
Neutral: Spider Changes:
-Spiders and Cave Spiders no longer take fall damage.
-Spiders now have a chance to spawn as Cave Spiders throughout the Overworld (15%).
-The chance for a Spider to spawn with a potion effect has been increased (25%).
-Both spider types will now move much faster than before (0.3 -> 0.35).
-Spiders and Cave Spiders no longer drop string, which makes crafting a bow early much tougher.
-The chances for a Spider to turn into a Spider Jockey have been increased (10%).
Hostile: General Info:
-Hostile Mobs will now spawn at light levels of 10 or less (Meant to make torches less effective in preventing them from spawning).
-The percentages of the types of armor worn by certain mobs has been changed:
Gold - 10%
Leather - 16%
Chain - 30%
Iron - 36%
Diamond - 8%
-The majority of hostile mobs will now be able to see a player from 30 blocks away instead of 16.
-Undead mobs will now always have the ability to pick up dropped armor and weapons, in addition to no longer being able to drop any equipment that they naturally spawned with.
Zombie Changes:
-Zombies will now pursue a player from 60 blocks away instead of 40.
-Regular Zombies/Zombie Villagers (Not counting Baby Zombies) are now a lot faster (.23 -> .27).
-Zombies that spawn in all types of biomes outside of deserts will now have the chance to become Husks instead (30%).
-Zombies now have a greater chance of spawning with weapons/armor as well as an increase chance of having enchantments on said equipment:
Armor - 5%>30%
Weapon - 15%
Armor Enchantments - 7%>75%
Weapon Enchanments - 5%>50%
-Zombies will now break down wooden doors at a much faster pace.
-Husks now have more health (20 HP -> 25 HP) and deal slightly more damage (4 HP -> 6 HP).
-All types of Zombies on land now have a 15% chance to spawn riding a tamed Zombie Horse.
-The default and maximum chance for a Zombie to call in reinforcements has been increased (Default:3%, Maximum:20%).
Skeleton Changes:
-Skeltons will no longer drop bows when killed.
-Skeletons will also have the same increased chances as Zombies when spawning with armor, armor enchantments, and weapon enchantments.
-Skeletons outside of icy biomes now have the chance to become Strays instead (30%).
-Normal Skeletons and Strays that spawn in the Nether will now always spawn as Wither Skeletons with bows instead.
Wither Skeleton Changes:
-The chance for a Wither Skeleton to drop a Wither Skull has been increased (2.5% -> 6.5%) for the sake of less grinding needed to obtain skulls.
Ghast Changes:
-Ghasts have increased health (10 HP -> 25 HP).
-A Ghast’s Fireball now has double the explosion power (EP - 1 -> EP - 2).
-Fireballs from a Ghast can also no longer be deflected, and will instead immediately explode when colliding with an arrow or when a player tries hitting it.
Blaze Changes:
-Blazes now shoot 3 Ghast Fireballs instead of 3 Blaze Fireballs (Each has an Explosion Power of 1). In addition, these Fireballs cannot be deflected.
-A Blaze's melee attack will now automatically set a player on fire.
Creeper Changes:
-Creepers now have shorter fuses (30 -> 25).
-Creepers can now spawn throughout the Nether.
-Creeper explosions will now produce fire similarly to Ghast Fireballs.
Enderman Changes:
-1-3 Endermites will now spawn after an Enderman dies.
-Endermen will no longer attack Endetmites.
-Endermen can now drop 0-2 Ender Pearls instead of just 0-1 (Meant to make grinding for pearls less time-consuming).
Endermite Changes:
-Endermites are now immune to damage from the clouds emitted by Dragon Fireballs and the Ender Dragon’s Breath Attack.
Witch Changes:
-Witches now have slightly increased health (26 HP -> 30 HP).
-When drinking a health potion, 1-2 Silverfish will spawn next to the Witch to attack nearby players.
Slime/Magma Cube Changes:
-All types of Slimes and Magma Cubes will no longer take fall damage.
-Each Slime/Magma Cube variant is now increased in size:
Small -> Medium
Medium -> Big
Big -> Huge (Size 4)
Guardian Changes:
-Can now spawn in all types of water throughout every biome, similar to Squids.
Boss Changes:
-Elder Guardians and the Ender Dragon now have increased health:
Elder Guardian - 80 HP -> 100 HP
Ender Dragon - 200 HP -> 300 HP
Ender Dragon Changes:
-Dragon Fireballs now create an explosion upon impact (EP = 2) in addition to emitting damaging purple clouds.
-For each use of the Breath Attack, the Ender Dragon will now also spawn 3-6 Endermites out of it's mouth to attack nearby players.
-Movement speed of the Ender Dragon is slightly increased (7.0 -> 7.5).
Wither Changes:
-The explosion created at the beginning is now much stronger (EP - 10).
-Occasionally, the Wither will lob an ignited TNT block at it's given target.
-When at half health, 3 Wither Skeletons in full iron armor will spawn next to the Wither.
I'm not sure how possible this is for modders, but for awhile now I was inspired by the "Extra Hard Mode" Bukkit plugin to make my own idea for a mod that changes pre-existing features to make survival Hard mode more difficult. In order to make it stand out, I tried coming up with some changes that are more unique/interesting than just making all mobs have more health/damage. Granted, that doesn't mean there aren't some buffs to mobs, they're just not quite as intense as other mods that try to increase the game's difficulty. You can see the list of changes I've come up with here:
Minecraft Delta Hard/Deltacore Mod (1.12)
Block/Item Changes:
-Bonemeal now only boosts a crop by one stage per usage.
-Beds will no longer skip the night, but will still set your spawn-point.
-Diamond Pickaxes can no longer be used to obtain Obsidian blocks (Elaborated on more in Overworld Changes).
-In order to prevent the lava-water Nether Portal trick, buckets will no longer be able to pick up lave source blocks.
-Tree saplings can no longer be obtained from leaf blocks (Meant to prevent tree farming).
-Seeds can no longer be obtained from grass, making it more difficult to obtain crops early.
Overworld Changes:
-All ores except Emerald and Diamond now have smaller veins.
-Silverfish block veins will now naturally spawn underground in any biome, in addition to spawning more frequently (Spawns at the same layers and frequency as iron ore).
-Obsidian blocks can now instead be found in 1-2 stacks within Dungeon (21.8%), Mineshaft (16.9%), Desert (23.5%) and Jungle Temple (15.3%) chests.
-Obsidian blocks in Village Blacksmith chests are now more rare (25.6% -> 16.2%).
-Dungeons will spawn more frequently throughout the Overworld.
-Dungeon spawners have now been reworked to include additional mob spawners:
Property Changes:
-The difficulty setting will now be locked by default, with the name of the difficulty being “Delta Hard” (When playing in Survival) or “Deltacore” (When playing on Hardcore Mode).
-Dirt, cobblestone (Not counting mossy cobblestone), wooden stair, and wooden plank blocks are now all affected by gravity similar to Sand blocks. In addition, the slab variants for wooden plank and cobblestone blocks will also be affected by gravity (Meant to make building a shelter and spelunking more difficult).
-Fire spreads much more quickly now.
-Lighting bolts will now create a fire explosion when striking the ground (Explosion Power = 2).
-Golden Carrots and Glistening Melons now require Gold Ingots in their crafting recipes instead of Gold Nuggets.
Mob Changes:
Passive:
-Domestic mobs that produce food will now spawn much less frequently throughout the Overworld.
-More expensive items are now necessary when breeding specific mobs:
Neutral:
Spider Changes:
-Spiders and Cave Spiders no longer take fall damage.
-Spiders now have a chance to spawn as Cave Spiders throughout the Overworld (15%).
-The chance for a Spider to spawn with a potion effect has been increased (25%).
-Both spider types will now move much faster than before (0.3 -> 0.35).
-Spiders and Cave Spiders no longer drop string, which makes crafting a bow early much tougher.
-The chances for a Spider to turn into a Spider Jockey have been increased (10%).
Hostile:
General Info:
-Hostile Mobs will now spawn at light levels of 10 or less (Meant to make torches less effective in preventing them from spawning).
-The percentages of the types of armor worn by certain mobs has been changed:
-The majority of hostile mobs will now be able to see a player from 30 blocks away instead of 16.
-Undead mobs will now always have the ability to pick up dropped armor and weapons, in addition to no longer being able to drop any equipment that they naturally spawned with.
Zombie Changes:
-Zombies will now pursue a player from 60 blocks away instead of 40.
-Regular Zombies/Zombie Villagers (Not counting Baby Zombies) are now a lot faster (.23 -> .27).
-Zombies that spawn in all types of biomes outside of deserts will now have the chance to become Husks instead (30%).
-Zombies now have a greater chance of spawning with weapons/armor as well as an increase chance of having enchantments on said equipment:
-Zombies will now break down wooden doors at a much faster pace.
-Husks now have more health (20 HP -> 25 HP) and deal slightly more damage (4 HP -> 6 HP).
-All types of Zombies on land now have a 15% chance to spawn riding a tamed Zombie Horse.
-The default and maximum chance for a Zombie to call in reinforcements has been increased (Default:3%, Maximum:20%).
Skeleton Changes:
-Skeltons will no longer drop bows when killed.
-Skeletons will also have the same increased chances as Zombies when spawning with armor, armor enchantments, and weapon enchantments.
-Skeletons outside of icy biomes now have the chance to become Strays instead (30%).
-Normal Skeletons and Strays that spawn in the Nether will now always spawn as Wither Skeletons with bows instead.
Wither Skeleton Changes:
-The chance for a Wither Skeleton to drop a Wither Skull has been increased (2.5% -> 6.5%) for the sake of less grinding needed to obtain skulls.
Ghast Changes:
-Ghasts have increased health (10 HP -> 25 HP).
-A Ghast’s Fireball now has double the explosion power (EP - 1 -> EP - 2).
-Fireballs from a Ghast can also no longer be deflected, and will instead immediately explode when colliding with an arrow or when a player tries hitting it.
Blaze Changes:
-Blazes now shoot 3 Ghast Fireballs instead of 3 Blaze Fireballs (Each has an Explosion Power of 1). In addition, these Fireballs cannot be deflected.
-A Blaze's melee attack will now automatically set a player on fire.
Creeper Changes:
-Creepers now have shorter fuses (30 -> 25).
-Creepers can now spawn throughout the Nether.
-Creeper explosions will now produce fire similarly to Ghast Fireballs.
Enderman Changes:
-1-3 Endermites will now spawn after an Enderman dies.
-Endermen will no longer attack Endetmites.
-Endermen can now drop 0-2 Ender Pearls instead of just 0-1 (Meant to make grinding for pearls less time-consuming).
Endermite Changes:
-Endermites are now immune to damage from the clouds emitted by Dragon Fireballs and the Ender Dragon’s Breath Attack.
Witch Changes:
-Witches now have slightly increased health (26 HP -> 30 HP).
-When drinking a health potion, 1-2 Silverfish will spawn next to the Witch to attack nearby players.
Slime/Magma Cube Changes:
-All types of Slimes and Magma Cubes will no longer take fall damage.
-Each Slime/Magma Cube variant is now increased in size:
Guardian Changes:
-Can now spawn in all types of water throughout every biome, similar to Squids.
Boss Changes:
-Elder Guardians and the Ender Dragon now have increased health:
Ender Dragon Changes:
-Dragon Fireballs now create an explosion upon impact (EP = 2) in addition to emitting damaging purple clouds.
-For each use of the Breath Attack, the Ender Dragon will now also spawn 3-6 Endermites out of it's mouth to attack nearby players.
-Movement speed of the Ender Dragon is slightly increased (7.0 -> 7.5).
Wither Changes:
-The explosion created at the beginning is now much stronger (EP - 10).
-Occasionally, the Wither will lob an ignited TNT block at it's given target.
-When at half health, 3 Wither Skeletons in full iron armor will spawn next to the Wither.
Criticism is always welcome!