New Biomes
Arkatron

If you like underground biomes, this thing is for you.

Minetest Game Random Content
AirplaneMan

Adds random content with no particular theme

Quartz Geodes
AwesomeDragon97

Extends the Geodes mod with quartz geodes.

Alien Tools
chaosomnium

Adds Alien Tool, Shield, biome, etc.

Sun Flowers
Hybrid Dog

This mod generates flowers which close their heads at night.

TerraContent Crimson
AirplaneMan

Introduces the Crimson to your worlds.

World Borders
argyle

Add impenetrable barriers to the sides and bottom of the world.

Noriel Sylvire's Minerals Core Modpack
Noriel_Sylvire

An API for automatically registering recipes, nodes and ore generation.

TerraContent Corruption
AirplaneMan

Adds the Corruption

Dirt Tunneler
Neuromancer

Makes dirt deeper and you can tunnel through it just by walking through it. Press spacebar/shift to tunnel upward/downward with auto placed ladders.

Dragonbone Stuff
Robot10&Scheletor

Adds dragonbone set

golden lava
Giga_M

add golden lava

Trucraft
mt-mods

An alternative crafting system done entirely in the world.

Purple ore
X-DE

Adds new purple ore to floating islands and caves from height 200 to 31000.

Underground Survival Lighting
Arkatron

If you like underground light in survival, use this mod, it can help that.

Spawn redo
cathaya7d4

Drop-in replacement for default/spawn for more third-party biomes

Sticks Stones
MikeRedwood

Makes Sticks from leaves and scatters small stones

Beautiful Boulders
regulus

Procedurally Generated Rocks with (pretty) Accurate Collision Boxes

Obdy
Sires

Adds some pretty OP tools :)

treasures
ademant

Treasure nodes with useful items

Floof
Momo

Add BicBiomeCraft inspired biomes, tools, and items.

Mud
Blocky Player

Adds stylish mud blocks and even blocks you can walk through. Setup traps, secret rooms and much more!

Silver
Jackknife

adds silver ore and tools

Too many ores
TeufloV

This adds 53 new ores. But most of them are useless.