I have ideas for either ‘biomes’ or specific areas of the map. Possibly the overall name too:
Hel (The second l is for losers.)
Shadow Caverns
Mushroom Under-Plains
Abyssal Canyons
Forgotten Ruins
Ideas in the past for ‘Nether’ Mutantion trees. Probably extra helpful in some cases down here.
Details of each:
Hel: Filled with lots of lava rivers and lakes. The inhabitants have mutated to allow survival in the intense heat. When in this area, it is extremely hot, but to give the player a reason to go in there, there could be special rare loot or something. One thing is possibly a thermal plant which contains heat-resistant gear and mutagen relative to that. Maybe Bionics that allow resistant to heat as well. Also flamethrowers. Because fire.
Shadow Caverns: Crap tons of those shady zombies. Zombies miners which provide a mining helmet, potential batteries from them, and a pickaxe if killed. However, mining is hard work and thus they have strong muscles despite the rot (Imagine a slightly weaker version of a zombie brute). Pools of sketchy-looking water appear on occasion, which totally don’t have mutated water-related creatures.
Mushroom Under-Plains: A wide space of underground land. Pillars of stone in various places hold up the ceiling. Don’t destroy them, it’s a bad idea. Glowing mushrooms of different colors and sizes light up parts of the caves. Acid Pools sit around, and acid related zombies and/or fungal monsters are plentiful. The fungal enemies, unlike above the world, are much more territorial, and don’t expand as much. Rare ruins will sit around, providing useful shelter and some strange and potentially odd loot.
Abyssal Canyons: Stretches of canyons, some big and some small appear on occasion. Don’t jump down, since their might not even be a bottom. Partially broken wooden bridges cross the canyons. Sometimes has a ‘toll house’ nearby, where only a few randomized zombies spawn inside.
Forgotten Ruins: Along your travels you may find what appears to be the towns of a forgotten people. Whether they were human or not, you will never know. However, while loot is plentiful, so are zombies. The thin alleys and streets, rubble from cave-ins from long ago, and other blockades limit you ability to escape if you walk into the wrong place.