The nether should have a few more biomes and structures.

First off, netherrack needs to be darker. And the lava in the nether should be red in the hell biome.

In some parts of the nether there should be a 'Frozen Hell' world, which is filled with blocks and mobs that slow you down and the weather is always foggy and harsh winds frequently strike. Water can exist here but requires a lot of light or else it will freeze immediately. The Zombie Pigmen here are a bit less 'mutated' and are more civilised like eskimos. It is however, impossible to trade with them, hitting them once is okay, but repeatedly attacking one will result in your death. The blaze in this biome is instead an opposite with their floating arms instead making them look snowflake shaped that shoots slow, but powerful projectiles thay gives you Slowness. Harsh wind comes with groups of strays and getting struck by the wind without getting shelter will result in slowness 2 along with a new effect called frostbite, which makes you more susceptible to damage. A new mob called Spirits inhabit this area and frequently curses the player with things like hunger, mining fatigue, or blindness. They appear when you destroy a 'Soulsand Monster Egg'.

After this is a revamp to nether fortresses. They now have parts that hang from the ceiling.

Another new biome is a volcanic basalt land with black coloured stone. The lava here is like normal and isn't red. There is frequent puddles of lava here with magma blocks strewn around them. Partially exposed bones also exist. Wither Skeletons spawn here very frequently and there are no Ghasts. Pigman also inhabit this area. There would be many volcanic based mobs here, like mobs that hide in lava and jumping at the player like lava spitting from a volcano. Trying to place water in this biome results in a very big sizzle sound and large amounts of smoke particles indicating it immediately evaporated into steam. This in PvP situations could work like smoke bombs, obfuscating names. There are dungeons that hang from the ceiling made primarily out of magma blocks and looks like beehives. They are inhabited by full of magma cubes and contain small 'demon' creatures that attack the player. Vexes also spawn sometimes around the area.