Cave Generator by PersonTheCat
A powerful and customizable modification of Mojang's overworld cave generator.
CrazyOres Core by Andy608, ISQUISHALL
CrazyOres Core acts as a base mod, that allows you to add different types of packs into your game. This opens up a variety of different gameplay depending on what packs you have installed!
Hardcore Ender Expansion by chylex
Hardcore Ender Expansion offers unique, new content across the End. The dragon became a truly challenging boss with a great reward, but the adventure doesn't end there. The End is no longer just an island. Go investigate unique ores, battle new mobs, travel to new biomes, the death awaits you!
dimension worldgen dungeons survival hardcore
Difficulty Tweaks by Lothrazar
Adds a bunch of small tweaks to the game that are all aimed at increasing the difficulty of the first few days. You cannot punch trees; you must turn saplings into sticks, and sticks into a damaged wooden axe. All stone tools require smoothstone instead of cobblestone. Furnace requires one coal in center to craft (so you may not get a furnace or stone tools on your first day; try making wooden tools and finding food first). You cannot gather dirt, grass, sand, clay, or gravel by hand (you can still break them by hand, but need a shovel to harvest the block). Sleeping in a bed will drain your hunger down and also gives you the rotten flesh hunger effect.