kekkres, I wanted to make Skaven with 3 unique features:
- Clans that cause infighting. 4 in total, controlled by castes, that do not like and debuff each other.
- Heavy debuff for being outside, bonus underground. Like reverse-cave-adaption, aka "no aboveground settlements/fighting".
- Joining clans means a random (?) civ-member is murdered, to prove their commitment.
Thats different from Goblins.
Perhaps, to capitalize upon this, you could go whole hog and add Clan-specific buildings? Like Clan Skryer would have a far more mechanical feel to them, with unstable (but hard hitting) muskets, flamethrowers, and the good old Ratling Guns, as well as a drill/minecart for mining... or combat if you can rig it right, higher quality (than usual for the clans) metal smelting and crafting... and overall just more uses for Warpstone by fueling ALL of this, and whatever else you can come up with. Lots of !FUN! here, due to the vast risk/reward dynamic. All of their stuff is super useful... but a large chunk of it could explode if not maintained well enough.
Of course, then we have Clan Moulder... kidnappers and masters of flesh. They would use Warpstone and Captives to essentially summon big freakin monsters like the Rat Ogre or the Hell-Pit Abomination for military and industrial uses. Also better bone and leather crafters, and farmers. Better farmers and animal trainers. Of course, these beasts might decide to stop being tame, and use their great strength to ensure that !FUN! is had.
Clan Eschin is all about sneaking and murdering... and unless you can set-up a stealth system in Fortress Mode... I would suggest scraping them, they are Ratman ninjas after-all. Replace with the Stormvermin, (there was a clan that had vastly more numbers of Stormvermin... don't remember the name though) bigger, stronger ratmen with black fur. Better for your military by virtue of actually being worth something in a fight without Warpstone use. Could also be better miners than standard as well due to their strength. Not too much !FUN! but... Skaven are notorious for their hatred of the words Kindness, Loyalty, and Mercy. Keep them happy, or they might start butchering anybody with the wrong fur colour.
The fourth great clan is of course, Clan Pestilence. Best way to use them would be to use syndrome reactions for poisoned weapons. They collect the worlds most deadly plagues... and toss them at people in glass balls, creating an AoE poison attack. They also use Warpstone, and put them in censors (incense burner-balls for religious rituals) and use them as flails. The Warpstone inside the thing causes poison effects, and will eventually kill the user. They are better Priests, as vastly more of their society is caught up in worship. Use Warpstone to fuel rituals so that you can get some bonuses... with some drawbacks. Could replace with the Grey Seers, Skaven Mages... who also might just explode at any time... and eat their students... unless said student eats them first.
Speaking of cannibalism... they are cannibals, a farming project can succeed by lessening the amount of mouths to feed, and increasing the amount of sentient food people rampaging through the warrens because of this fact. This would be the most !FUN! race to play, lots of things to experiment with, lots of potential for tantrum spirals, and unending amounts of fluffy backstabbing.
To be honest... it sounds like the epitome of Dwarf Fortress.