I feel like the caverns can get kinda boring. They're pretty much the same every time, and there's little difference between the 3 levels, except for increasing difficulty. The surface is more fun with its many biomes, so how about biomes like that below ground? Here are some ideas.
Mushroom Jungle - Like we have now, maybe more exaggerated, denser mushroom packing and taller mushrooms, like an underground highwood.
Slime Jungle - Similar, but the dominant "flora" are
slime molds or fantasy slime instead of mushrooms. The slimy "trees" do not yield wood when harvested, but some other resource, maybe a form of wax or silk.
Orewood Forest - A more fantasy type of forest where the trees have evolved to absorb rock and grow into ironwood, bronzewood, goldwood and so on. These trees might require picks to harvest instead of axes.
Petrified Forest - A forest made of stone. Could leave petrified wood,
agatized wood,
opalized wood, and so on. The trees are not actual trees, but stone, and thus will require picks instead of axes and might cause cave-ins. There might also be creatures made of stone, or creatures with stony hides or stone-like camouflage.
Crystal Garden - A cave made up of enormous crystals.
Like this. Could be that the crystals are actively growing, like trees, like one of those kids science experiments, and can be harvested and regrown by the dwarves.
Rivers, Lakes and Oceans - Large bodies of water with unique organisms. Underground oceans could be filled with salt water and filled with gigantic oceanic creatures like sharks, whales, squids and mosasaurs. These might be generated with no air at all, just rocks and then water.
Fossil Ocean - The remains of an ancient surface or underground ocean that has slowly dried up. Petrified corals, sponges and shells liter the ground. A few salty pools are all that remain of the water. The water disappeared slowly enough that some sea creatures were able to adapt to a life on land, monster crabs, walking sharks, land kelp, etc.
Deep Earth Vent - A habitat like a deep sea vent, but above water. Or partially above water, like a swamp. Filled with deep sea vent creatures, tube worms, blind crabs, bacterial mats, vampire squids, etc, and some fantastical versions of them. The volcanic vents belch hot smoke and poisonous fumes, deadly to dwarves but necessary for the vent life.
Tendril Savanna - A biome similar to grasslands on the surface, but the "grass" is actually sessile worm-like creatures which extract nourishment from the rock and mud. Like savannas on the surface, the Tendril Savanna is home to large and dangerous animals.
Underground Desert - Like deserts on the surface, a barren biome filled with sand and scorpions. Maybe some extra hazards like quicksand or sand "waterfalls". Hot spots and cold spots underground could cause temperature and pressure variations generating wind, with sand storms and dust devils. Or creatures that can swim through the sand as if it were water.
Fire Pit - An underground fossil fuel deposit. Pockets of methane belch flame and noxious vapors into the air. Petroleum tar oozes through the rock into rivers and lake. Boulders of coal litter the ground. And the plants and wildlife that make the Fire Pits home have evolved to lead a high octane life, many capable of belching flame.
Tar Swamp/Slime Swamp - A swamp-like environment, but instead of water, it is filled with petroleum tar or glowing green fantasy slime. The slime might be some kind of acid that starts to dissolve any creatures caught in it and allowing acid-loving microbes to dissolve them and form a unique ecosystem. Tar swamps would be like real life tar pits, creatures get stuck in them, and predators and scavengers try to finish off the remains without getting stuck themselves.
Ancient City - And unnatural cave, shaped of rectangles and circles, houses and streets deep underground. A relic of an ancient civilization or one of Armok's aborted creations. The empty halls are filled with creatures that have come to settle in the ready made shelters, or perhaps by the undead souls or automated guardians of the long dead civilization. These cities might look like
Derinkuyu.
Lava Tubes - A cave made up of tubes, rather than wide open spaces, like
real-life lava tubes. A lucky (or unlucky) dwarf might find some active magma. I imagine a series of mostly horizontal tubes might radiate from a central vertical shaft.
Prehistoric Pocket - A cave filled with dinosaurs and other prehistoric fauna.
Eryops,
Eurypterid,
Meganeura,
Sebecus,
Terror Birds,
Rhamphorhynchus, etc.
Magma River/Swamp - A classic fantasy/VG "fire stage" where magma flows like rivers or collects in pools like a swamp. Creatures from the magma sea can be found there.
Root Cellar - A forest of "trees" that grow from the ceiling. The floors might be barren and uneven, or littered with some sort of vile substance, necessitating the evolutionary shift to ceiling dwelling. Animal life is filled with things like manera and troglodytes that can climb the walls and branches, as well as lots of flying animals like swallows and bats.
Mega Ant Nest - An artificial cave made by a colony of giant ants. These might exist solely on their own or connect to one of the other cavern types. The ant nests are full of purpose built chambers for specific resources that the mega ants manage.
Fungal farms, nurseries for eggs and larvae,
slave chambers,
honey-pot taverns,
aphid pastures,
devil's gardens,
silk farms,
medicine sheds, worker barracks, soldier barracks, Queen's chamber or
gamergate chambers. Mega Ants might have more complicated casts like Carpenters and Masons. Dwarves can try to steal some of the wonders that the ants produce, but the ants will fight back. Ants are also capable diggers and builders, they might try to burrow into the dwarven fortress themselves, or wall off any entrances that the dwarves make into their realm.
Dragon Hive - An idea from
Legend of Dragon, the Green Tusked Dragon, Feyrbrand excreted a poison that causes all the trees and animals around it to mutate into deadly poisonous creatures. The toxic air can allegedly cause weaker individuals to become sick (or those without Dragoon Spirits). In Dwarf Fortress, this might be some sort of Necromancer-like effect that mutates lifeforms into dragonic versions that grow scales and secrete their own poison. There might be some particular giant beast like the dragon Feyrbrand which is the root cause to this, but I would point out that slaying Feyrbrand in LoD does not cause the Nest of Dragon to change in any way.
Subterranean Fortress - A subterranean fortress much like a generated dwarven fortress, but inhabited by deep dwarves, goblins, troglodytes, cave ogres or subterranean animalmen. This is obviously super dangerous, but a savvy player would be warned by finding constructed walls around the borders of this cavern type instead of stone. Possibly more akin to finding a curious structure underground than an entire layer. The subterraneans might worship some alien god from deep underground that can actively impact events. In a magical setting, your own gods might encourage you to find and destroy this deity. Destroying the temple or idol of the god could stop its power and possibly destroy the entire subterranean fortress. Maybe the inhabitants simply fall dead or turn to stone or run away, or maybe something Fun like the layer fills with magma or water or poison gas.
Carnivforest - An underground forest where the apex predators are all mutated plants that try to kill any animal that comes near them. Oversized venus fly traps, strangling vines that use corpses to fertilize their own roots, cordyceps that take over the brains of their hosts, etc. I love the ballistomycete from Dungeon Crawl, a floating orb of fungal spores that travels around the dungeon and explodes on contact with creatures or obstacles. New ballistomycetes grow from the spores spread by the explosion, perpetuating the cycle. There should also be valuable resources in the Carnivforest as well, like delicious nectar that can be turned into the best booze.