A new location: Black Dragon Island
Now that there are special locations where fire, ice and shock immunities are of use, to complete the quartet a place of acid could be added post-Steam. Here's what I thought:
Somewhere towards the left of the map there'd be a black dragon island. In order to make the bridge building skill more useful and worthwhile to develop, it could be on a lake of acid that won't freeze with the cold spell and obviously you can't swim across it. There would be two ways to get to the island:
1. Build a bridge
2. A cave system suitably dangerous to make the player consider carefully whether to risk it or build the bridge (if possible), with a final up staircase on the island.
Because of item destruction etc. the place would be for 30+ level characters anyway, which would also be good because the late game could use some special places. Quite often I've had to grind in the blue dragon caves just to get levels required for certain quests, so adding another location would make the late game more interesting. So it would be accessible only after a certain level due to its frightening nature.
The island itself would have another cave with a black dragon vault including wyrms, and a random artifact or two (or a particular one, if new ones are added, like a dragon tooth amulet that grants acid immunity and helps with critical hits and something else for spellcasters). And maybe in a secret room a guaranteed scroll of danger or a boar skull.