World Borders
argyle

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

Archaeology
SkyBuilder1717

Adds an looting and API in archaeologing

Trucraft
mt-mods

An alternative crafting system done entirely in the world.

Obdy
Sires

Adds some pretty OP tools :)

Underground Survival Lighting
Arkatron

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

Purple ore
X-DE

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

treasures
ademant

Treasure nodes with useful items

Floof
Momo

Add BicBiomeCraft inspired biomes, tools, and items.

Fly-B-Gone!
Hugues Ross

Hides butterflies/fireflies

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.