War Flag
Types: Treasure, Attack
Cost: $6
$3. When you play this, trash this or discard a Treasure and each other player discards a card for each coin in its cost, and then draws until they have 3 cards in hand.
In games using this, when you buy a card costing $5 or more, trash a card you have in play.
This feels a lot like Death Cart, which is to say that this isn't exactly a one-shot ability. You can (and probably want to) avoid trashing this by discarding treasures when you play it. You can still access the same ability without losing the card.
Examples include Mining Village...
If Mining Village, a Village that you can trash for economy, would count for the contest, how does War Flag, a variable Attack that you can trash for economy, not count for the contest?
Because Trashing this isn't exclusively what gives you the benefit. Ideally, you'll be discarding treasure for the benefit. When I say a one shot, I mean you can blow it up to get something that you otherwise can't get. You have one shot at that benefit. This card gives you several opportunities to have the same benefit over and over, rather than giving you one shot at it. Does that make sense? As I thought of this challenge I was worried it would be hard to explain.
There is no way to get $3 from War Flag without trashing it (because discarding Treasures effectively reduces the $ it produces), in much the same way that there is no way to get the +$2 from
Mining Village without trashing it.
The repeatable benefit of War Flag is an Attack that effectively costs $ to run. The one-shot is getting the $ and the Attack without paying for it otherwise.
I think you are undervaluing the ability to trash War Flag early to hamper other players' turns before you can build a consistent Gold-discarding deck. Even trashing a War Flag that you want to keep (when it misses a good Treasure) and rebuying War Flag is not a moot point because of its in-games-using-this effect. Because buying Victory card dismantles your deck, leveraging the War Flags you previously used for consistent Attacking for inconsistent economy is not to be ignored.
Would the following count?
Drunkard
Types: Action
Cost: $2
+2 Cards. You may trash this. If you do, +2 Cards.
It's a repeatable draw benefit that can be a bigger draw benefit once.
That's the same thing, conceptually.