Muspelheim. The land of fire giants. I have populated it with Magma cubes and blazes. It is mostly lava and stone, with some netherack.
The world tree Yggdrasil has many levels, and the one I am planning next is Niflheim, the Norse land of the dead. It is a cold world, so it will be made of ice, snow and iron, populated by iron golems and skeletons. Niflheim is ruled by Hel, a goddess that has half of her body dead and half of her body alive. In my Yggdrasil, Niflheim and muspelheim is located underground, the rest are found above it.
In Norse mythology, Muspelheim is the land of the Fire Jotun, Jotun means giants. They are ruled by Surtr, who is suppose to cover the earth in fire during Ragnarok.