I was looking at the Haven Fountain just now, and I thought to myself:
It would be nice if we could actually have water type dungeons.
And then it hit me.
A whole new water element, and a water type boss (like how Lord Vanaduke is in Fire stratum) (it would probably be classed as Fiend).
The boss would be completely unhittable, and the way to hit him is to turn on a cryogenics machine by... idk various switch?
And you could attack him only when he is frozen, and he wouldn't have the same weaknesses as Fiend, he would be weak to shadow and resistant to elemental? or probably piercing.
Idk, you discuss.
For a moment I was worried you wanted a new damage type, but a water-themed dungeon could be fine. Boss immune to damage unless frozen, with ways to freeze him on the level... fair enough.
And your token rewards shall be super soakers :O