With Ironmonger, we've cemented the idea that the prefix "Iron-" signifies a card that will give variable bonuses based on the type of some other card. Thus, the following ideas naturally present themselves:
Ironsmith
$5 - Action
Reveal the top card of your deck. If it is a...
Treasure, then copies of that card produce an extra $1 this turn.
Action, then copies of that card produce an extra +1 Action this turn.
Contraband of Iron
$5 - Treasure
Worth $3
+1 Buy
When you play this, the player to your left names a card. You can't buy that card this turn.
If the named card is a Victory card, +1 Card.
If the named card is an Action card, +1 Buy.
If the named card is a Treasure card, +$1.
Ruined Iron
$0 - Action/Ruins
+1 Buy
The player to your left reveals a card from his hand. If it is a...
Victory card, +1 Card;
Treasure card, +$1;
Action card, +1 Action.
Iron Seal
$6 - Treasure
Worth $2
--
While this is in play, when you buy a card, if it is a Victory card, +1 Buy; if it is an Action card, +$1; if it is a Treasure card, +$2.
While this is in play, when you gain a card, you may put that card on top of your deck.
Tribute of Iron
$6 - Action
The player to your left reveals then discards the top 2 cards of his deck. For each differently named card revealed, if it is an... Action Card, +2 Actions; Treasure Card, +2 Coins; Victory Card, +2 Cards.
If the bonus you got from this card is...
+2 Cards, then +$1
+4 Cards, then +1 Buy
+2 Cards, +2 Actions, then +1 Card
+2 Cards, +$2, then +1 Action
+2 Actions, then +1 Action
+4 Actions, then +1 Card
+2 Actions, +$2, then +$1
+$2, then +1 Action
+$4, then play this again.
Ironcutter
$5 - Action
+1 Buy
+$2
If this card just gave you +1 Buy, +$2, then +1 Card.