I was mostly interested in whether or not such Kingdom cards could be good enough to exist, but since people seem to be so interested in particulars:
Blockhouse
Types: Victory
Cost: $6
Worth 1VP for each Attack in your deck.
You can't buy this unless another player played an Attack on his previous turn.
(VP value is not finalized) (it might be changed so player's cannot gain it, rather than simply cannot buy it)
Just add a line saying "Setup: if there are no Attack cards in the Kingdom, add one and play this game with 11 Kingdom cards."
A setup clause is going to look messy on the thing regardless because it already has a horizontal rule to separate its value from its "can't-buy" condition. Even if it didn't have that problem, inserting a setup clause to add an Attack to the Kingdom is less than desirable because I don't want to stamp on
Young Witch's identity and I would not like 12-card Kingdoms to be a thing. A setup clause would really need to remove itself and replace it, but that strikes me as odd.
make a kingdom card that is a weak, simple attack, and include your card whenever that attack is in the kingdom. That way you avoid cluttering the original card.
I have considered having another card that adds Blockhouse to the Supply instead. That still has the 12-card Kingdom issue. Even without that, the Attack cannot be something simple just so that Blockhouse can be used since
other players have to play Attacks for you to be able to gain Blockhouses. Even if the Attack were made good enough to be worth playing around Blockhouse, it would make the play patterns of Blockhouse less dynamic since there would be an obvious synergy with the extra Attack pile.