I wanted to do something cool with Aqua Regia as a Potion cost card. An acid so named because it could dissolve the "noble" metals - Platinum and Gold). As far as I know, there are no cards that really talk about Platinum, since they aren't always in the supply. And then whose treasure would it trash? Yours for some crazy benefit, or theirs?
Was thinking of having it trash all treasures it finds, like a Thief that you don't get to keep the loot from, but then realized that would get degenerate fast if multiple people used one. At least with a Thief or Noble Brigand, somebody has the treasure for at least a little while. And then of course Pirate Ships become their own Treasure-less force.
I fought with that idea for a while, but never got it close enough to even salvage an idea like zahlman did with his own. Plus you know, with hidden names, the fact that I was using a historical potion name won't help it. Also thought about something for Aqua Fortis (another acid that can dissolve silver but not gold) or Aqua Vitae (basically booze).