Some stuff that could easily be added in vanilla df by a modder. Some of these are probably already in masterwork but should be looked into if they arent imo.
Being able to craft your own armor in your own size with materials (a lot of material for bigger sizes). This uses armorsmithing/weaponsmithing of course. Now that you can cut down trees you could potentially have a complex set of reactions with wood. Such as smelting down armor to get out bars. Use bars with wood to get armor. This being ineffecient way of doing it outside of a smelter/forge it could use up more materials and bars than usual.
Would be nice to have the option to add several diffrent size civs, so that you could potentionally find armor in your size as a animal man of other size than normal (without having to custom make it in a fort and then find the fort and hopefully find your armor somewhere in the complete mess the AI does). Also would make it more fun to find elephant men opponents with armor. They could obviously breed slower or something like that to balance world power. These civs could also use weapons that fit with thier size. A shortsword is pretty small on an elephant man.
More variants of playable animal men. So many rarely used tokens and it should be easy to custom make few creatures so that they are a bit easier for newbies or harder of course. Easier creatures could easily be added as an option to play with liberal variants of added tokens such as [NO_EAT], [NO_DRINK], [NO_SLEEP], [LOW_LIGHT_VISION:10000], [FIREIMMUNE], [FIREIMMUNE_SUPER], [NO_DIZZINESS]. [NO_FEVERS], [NO_THOUGHT_CENTER_FOR_MOVEMENT], [NOEXERT], [NONAUSEA], [NOPAIN], [NOSTUN], [NO_CONNECTIONS_FOR_MOVEMENT], [PARALYZEIMMUNE], FIXED TEMP, [SUPRAVISION], webbing, regenerating tissues, steel bones, [PRONE_TO_RAGE:?]. These creatures could also make the adventure much more interesting since they would be a harder opponent to fight if allowed to be in larger numbers in civs.
If you want a harder adventure you could easily add creatures that learn slower or dont even get increased physical attributes.
More odor strings and SOUND.
More creatures with diffrent colors of blood. You really see a lot of blood in the game as an adventurer...
If the creature has a lair, adding many diffrent types of LAIR_HUNTER_SPEECH is a must. Specially in mazes.
More GLOWTILES.
An easy option to remove boogeymen and such, not everyone likes having them in the game. Its not like sleeping alone outside isnt dangerous enough as it is even as a legendary skilled fighter.
More rare types of vampires (remember an old one in a mod that had a hidden probuscus which it used to infect other intellingent creatures with, it also had a reaction attack with poison spit or something like that). Bizarre but fun vampires to expose.