Lava Lakes and Fire Shrines

If a large cave forms deep enough, it should form a lava lake! If the area is a slime chunk, magma cubes spawn instead of slimes, but there are no other nether mobs. Small deposits of magma may spawn at the edges of the lava. There should be a "shelf" or "shore" around most lava lakes to give the player somewhere to walk. Magma might get in the way sometimes, but you could always build over it with cobblestone.

If the cave somehow fills with water, the lava is entirely replaced with Magma, making the cave terribly dangerous to traverse without water breathing and fire resistance potions.

Players may hunt magma cubes in this biome, and since it's a very deep cave, diamonds may appear in the walls. The light from the lava may also provide a brief rest from monsters.

There's a chance of spawning a "fire shrine" structure here, built into the wall of the lava lake cave. It has a ruined portal and some other nether blocks like basalt bricks. The shrine contains a treasure chest and is also an archaeology site, yielding nether-themed archaeology items.