Which units would you replace in the Dark Elf faction?

That wouldn't fit the spirit of the Dungeon. The Dungeon is a place for terrifying creatures, and those that spit poison belong somewhere else, like in a swamp.

Well, maybe not spitting poison exactly, but we could have a terrifying creature from the Dungeon that deals poison damage (I don't know what kind of attack, it could be non-ranged, so it can stay at a distance and attack). I just think that having only Stalkers might not be enough; the Dungeon needs some other poisonous creatures.