Volcanoes and Magma Golems

Volcanoes will have a large amount of lava, magma blocks, and a material resembling netherrack.

They would have two types:

Active: Lots of lava. Incredibly hot temperatures.

Dormant: Not much lava. May spawn in any biome, including cold ones.

Magma Golems can also be added along with them. They would shoot fireballs, make bubble columns when underwater, and be instantly damaged by any snow/ice.