Extensive adds a bunch of new biomes, blocks, mobs, and items to your world. You may stumble on the molten plains covered in solid volcanic rock and peppered with molten trees. Here you can harvest molten rock and wood for new blocks to build with. As the biome name suggests, it is full of lava so come prepared!
Or you could get caught up in the frigid pure water wasteland covered in a variety of icy blocks and wood. This biome is full of frozen mobs and ruins.
There’s also the blood snow forest with its enchanting pink aura. It’s also riddled with cherry wisps floating up and down throughout the trees.
And there’s even a new dimension accessible via frozen obsidian blocks and frozen flint and steel called the Achromic Dimension. You essentially cover normal obsidian and normal flint and steel with ice to craft the necessary parts. And the biome itself is a frozen series of caves and cliffs but filled with liquid dry ice that will burn on contact. It’s essentially the inverse of the nether.
Each new biome has its own mobs as well like the zombie mage, cherry wisp, and ghostly stray.
And of course with new ores to be found, this mod also adds new armor sets and tools/weapons that each have their own buffs when wearing full sets.