I know there have been several suggestions regarding this, but here is my spin.
Glass building pieces, formed out of steel reinforcements, glass and composite obsidian (like 1 or something)…or even just raw obsidian, you might throw alchemical powder in it, or dragonpowder for tempering. Only make walls, doors, ceilings, and roofs, no fences or foundations…or maybe fences, but no foundations.
Another thing, is why don’t we have iron bar walls and ceiling pieces? It would really go well with the slavery thing. Maybe a DLC pack for dungeons, slavery and cannibals. Placeable shackles on the wall for look. I’m not suggesting how far some of these modders have gone with these two concepts, but some of the more basic ideas.