So for some reason, I was thinking about what a Lord of the Rings Dominion expansion would look like. And the idea that came to me was this:
Ring of Power
Treasure: cost $0
Worth $3
When you play this, gain a Curse.
It's not a Kingdom card. Your starting deck consists of three Estates (or Shelters), six Coppers, and the One Ring. (Decide whether to use the Ring the same way you decide whether to use Shelters or Colonies: chance of using it is proportional to the number of cards from the expansion.)
This seems to avoid some of the problems that "When you play this, gain a Curse" cards have?—such as just being Gold when the Curses run out. That's still true for this, of course, but the fact that you have it in your starting deck means you can't just wait for the Curses to run out before gaining it, or not gain one if you don't think the Curses are going to run out, or anything. And its presence in your starting deck makes for some interesting tactical choices—at the most basic level, would you rather open Silver/Silver, or Silver/Gold/Curse?
I also thought of a Kingdom card that would interact with this well—
Hobbit
Action: cost $3
+1 Card
+1 Action
+1 Buy
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While this is in play, when you gain a card you may trash it.
—the idea being, if you have a Hobbit you can use the One Ring more safely... but its combo with Market Square and Goons is probably too good, so it should have a different top half than +1 Buy.