I'm considering an Ant Nest mod.
The whole thing would be tiny in the real world; a 16x16 would be comparable to a couple of dwarf-scale tiles at most. The entire world would be the sheltered courtyard of some Dwarf Fortress. Differing climates would be a result of sunlight levels caused by shade-bearing constructions. An ocean would be a pond. Mountains would be dirt patches. The desert is the result of someone spilling his 90% proof drink.
Using castes, only the Queen would be able to reproduce, with various smaller castes with abilities like thicker armour, acid-poison bites, extra-strong mandibles, and so forth. Of course the most common ants would be very weak. Using mandibles and limbs as weapons or graspers, ants could equip themselves with improved exoskeleton plates made from various materials.
Food would be obtained by ants the same way as dwarves. Fungi would be grown underground, plants aboveground, and various animals raised for food. Milking would also occur. Fishing could grant things like mosquito larvae. Alcohol would be available from fermenting various food products. Tiny creatures such as mites would make a delicious snack, or maybe a beloved pet.
Crafting would be slightly different. Bone would be quite rare but valuable, often having to be dug up, but shells and chitin would be plentiful. Stonecrafting can use the grains you dig up, and the stalks of 'large' plants and fungi provide for woodworking. Fibres from plants and silk from huge, unruly cave spiders provide for weaving. Behold as your jewelcrafters work exquisite salt crystals and fossilised ant eggs! Gasp as glassmakers polish and shape tiny grains of sand into shining relics! Feel pride as blacksmiths work slivers of material into hard and durable tools, useful for trading or cracking open the carapaces of your enemies!
On top would be the 'trees', which would actually be the stems of small vegetation. Clovers, grasses, mushrooms, and so forth. These could be used for wood and construction, and possibly food, depending on the plant. The 'soil' layer would be a thin layer of dust and rotten vegetation over the ground. Under that would be varying amounts of diggable and undiggable 'stone'. 'Boulders' would be large grains of sand or dirt, or very soft stone, usable in construction and crafting. Large and small clusters and veins, possible even layers of undiggable material would represent actual pebbles and rocks and other un-antable problems. Your tunnels would gain irregular, organic forms due to the unavoidable obstacles. (unless you dug out below the rock, and collapsed it, then filled in around it...)
There may also be material that represents eggs, roots and other subterranean formations. Clack your mandibles with glee as you find a maggot that will feed your nest for months, or panic wildly as a tuber releases poison that makes your ants die by melting from the inside! Or, perhaps you will find a vein of bone, which can be chipped away and used for valuable crafts!
The caverns layer could represent underground air pockets, including things like worm-tunnels and buried nests. Further down, the magma sea represents that ancient nemesis, groundwater. Or, if you're feeling exceptionally cruel, imagine your ants embarked on the floor tile above a tunnel full of magma. Those poor Dwarves. The adamantine pillars? Those are actually flaking iron nails or bolts discarded by dwarves. Lost in battle, carelessness or deceit, these hardened elemental formations provide ants with a legendary material for fashioning armour, weapons or crafts from. The mythical land below the deeps is made of nothing but impenetrable stone blocks with bottomless gaps! The cold and lifeless world below is home to the most deadly and twisted creatures known to Ant-kind, as well as dwarves.
The mighty ant nest would face many enemies. Termites, fire ants, spiders and other feral and terrifying beasts would regularly assault your solid-dirt walls, eager to eat your baby ants and steal your fungus supplies. Ant-Legends speak of huge beasts of the sky-world such as the "Small Spider" and "Tiny Frog" which are small enough to fit in your tunnels, all the better to devour your colony. Strange beasts from below the topsoil defy description, each Ant-Explorer seemingly picking random attributes and applying them to the subject of their story. They also say that touching the iron relics will result in hordes of sub-soil monsters appearing, but that's about as likely as Cordyceps unilateralis-infested 'Zombie Ants' attacking your nest and infecting everyone! Haha, that would be silly!
All in all, I think an Ant Colony would be fun. The similarities between dwarf and ant are too many to list, and with only a few thematic changes we could see a whole new microscopic world that could be occurring in a Dwarf Fortress.
I also had an idea for Bizarro Fortress, where elves were made of fire, adamantine explodes and alcohol is poisonous.
Edit: Oh dear Ant-God. The ultimate enemy civ would be zombie fire ants supported by their fly overlords. The thought is chilling.