There are some nice entries already! Here's a split pile:
Clue - Treasure, $1* cost.
$1
+ $1 per Clue you have in play. At Clean-up, return this to the Supply.
-
This can't cost less than $1. When you buy this, +1 Buy.
Hideaway - Night, $6 cost.
Gain a card costing up to how much $ you have unspent.
5 Clues on top of 5 Hideaways.
I think only one Hideaway is worse than Market Square or Candlestick Maker, except for never being drawn dead. So, it’s good only if you have two or more. However, it’s not so easy to buy two or more Hideaways because Clues are returning to the top of the pile.
If you gain two Hideaways and use them to gain extra cards that cost $6 or less, only after the third card gained this way you have some gain, since the first two you would buy anyway instead of those Hideaways (and already play them in the turn you would gain them with Hideaways). As the game probably would be more near the to end when you gain your second Hideaway, maybe there’s not much time to do the trick three times or more.
So Hideaway seems to me to be a kind of an expensive and hard to trigger Duplicate, specific for $7 cost cards, Provinces, Platina and Colonies. For $7 cost cards and Platinum, if you have one in play, it’s easier to gain another with Changeling.
Also, there's a multiplayer issue: only two players can have more than one Hideaway, which doesn't seem too fair for a card that does its best only in pairs or multiples.