Hi! (Again)
This is a mod that already adds a lot.
Name: Better Dungeons/Chocolate Quest
Link: http://hexxit.wikia.com/wiki/Better_Dungeons (One of the only ones that really has a description, because there is no official website)
Description: This mod adds: Many new and very large dungeons, Several new mobs, adds many bosses, blocks, items, weapons, tools, essences, etc, but the very focus are its huge new dungeons!
Thanks!