I'm seeing a LOT of mod request threads, many of them requesting things that cannot be modded in Dwarf Fortress. As a DF modding veteran, I figured it would be a good idea to make a post explaining what kinds of things you CAN mod and what kinds of things you CANNOT mod, both to inspire the creative and give requesters a reality check.


Things you CAN mod in Dwarf Fortress:

Almost anything related to creatures (both civilized and non-civilized), including body shapes, personality traits, multiple castes (each of which can be almost completely unique), natural magic abilities...loads and loads of stuff here. You don't need to play as dwarves at all.

Ditto for plants - how they grow, what kind of fruits they produce, what kind of resources can be extracted from them, etc.

Materials, including where they can be found, what their physical properties are, and what happens if you eat, touch, or get injected with them. Also, parts of plants and creatures can be made of any material.

Weapons, armor/clothing, and tools (projectile weapons are limited in how much you can change them, you can't really fine-tune firing rate for example, though you can make different kinds of ammo)

Workshops that can be built in-game

Reactions (essentially crafting recipes) that can be performed in said workshops, which can take pretty much any item or items and convert it into other item or items

Civilizations - what biomes they prefer, what kind of sites they build (though it must be one of the existing 5 types of existing sites - cities, underground forts, enchanted forests, dark fortresses, or caves), what their values and ethics are, what aspects of nature they worship, all sorts of stuff related to how their nobility works

Languages (that is to say the fictional language your creatures speak)

"Syndromes", which is basically the game's short-hand for "temporary or permanent status effects that can be applied to a creature to alter their base state". These are extremely fun and flexible, they can do anything from making a creature's skin rot off to turning them immortal or giving them the power to inflict a different (or the same) syndrome on another creature with magic. There are some limitations to magic in its current state (no teleports, ice is finicky, transformation always instantly heals all wounds, removing syndromes that do not wear off naturally is annoyingly still impossible) but you can have loads of fun here.

Divine secrets and curses, which basically apply a syndrome to creatures who worship/anger the gods. Secrets can also be taught to acolytes (but in practice this only happens if the secret incorporates some form of animating the dead, because only zombie-raisers will build towers and only towers will attract acolytes). However, your dwarves can learn magic if you get your hands on a magic slab or book.

Graphics and sounds (these are new and I don't know much about them yet)


Things you CANNOT mod in Dwarf Fortress:

Interface changes (aside from graphics)
Entirely new AI behaviors
Pretty much anything not mentioned in the above list


In a nutshell:
If it involves changing the game's world or setting, you can probably mod it.
If it involves changing the way you interact with the game, you probably can't mod it.

Feel free to ask any questions here, I've been modding this game for years and have a pretty good grasp of what is and isn't possible, but please do not waste time asking for things that are not in the above list of things that are possible to mod, because if it's not there, the answer is no.*

*Without DFHack or another unofficial hacking tool.