Avoiding of dfhack not give you new features. But using dfhack can add ability to poison weapons. And other intersting elements. Also I suggest more trolls. Servant tweaking is good, but goblins use not only trolls (in vanilla they also have ogres, blizzard men and maneras). As they shear trolls, they could milk ogresses or use ogres as mounts/pullers. Milking sapients needs dfhack script for work.
With scripts, you can add workshops for creating blizzard men and grimelings. For blizzard men creating player need cold temperature and snow/ice, for grimelings player need plants and grimeling will be made of this plant material.
Goblins as analogue of cats can have hungry head. As analogue both cat and dog goblins can have stranglers.
Harpies, blendecs and nightwings isn't slow learners, but can be heroes and you can make "domestication"/enslaving/teaching of wild harpies, blendecs and nightwings in dark fortress mode. What about milking harpies?
Demon leader probably can make underground elemantmen in workshops or as necromancer with transforming (for blood men or mud men).
Also goblins could corrupt stolen childred into night troll like species of servants.
I actually don't use dfhack when I play (unless I am using a specific mod that requires it or some sort of special playthrough) as it slows down my game and crashes often. So, I don't plan on making my Fortress mods reliant of dfhack. I think Meph was talking about creating a goblin mod with dfhack though to add new game mechanics.
I could add poisons without dfhack though. Arrows (of any material) and a poison could be turned into "poison arrow" at a workshop. The new arrow material could have the poison syndrome attached to it. I could do the same with a few other weapons (poison dart, poison dagger, poison blade, etc.) Or I could add an ability to altars, shrines and monuments to add a chance for a weapon made out of demonic/abyssal material to spawn from an offering. This demonic material would have a necrotic/poisonous syndrome.
Currently this mod has Trolls and Ogres as pets, but I will add manera and blizzard men to the next update. I'll also add WAGON_PULLER to trolls and ogres. Although potentially accurate in the goblin culture (and probably valid), milking sapient creatures like ogres for drinkable milk is too weird/gross (for my own personal views) to add to the game.
I will also add slow learner grimelings and make them a pet and change "worg" to ice wolf to keep with vanilla. I will also remove the aquatic tag (and add the no breathe tag) from sea monsters and add them as a pet. They will be very slow/immobile on land, but can be pastured in a pit (which can then be flooded for a sea monster moat/tank).
I'll add hungry head and strangler as pets too.
I'll add harpies, foul blenders, dark gnomes and night wings to the goblin civilizations. They will show up in the pet tab though similiar to the others (e.g. trolls, ogres, etc.) and can be bought/sold/pastured and help with menial labor (building/stockpiling).
I will add mud men and blood men with slow learner as pets too to go along with the blizzard men and grimelings.
I could add a unique poison that transforms creatures that drink it into demonic creatures or some sort of abomination. It wouldn't be unique to children though. You would have to pasture the creatures you want to drink it into a room with that drink in it and then lock the door. When the "pets" get thirsty they will drink it and transform.
Thanks for the ideas
these are good