Hmm, seems like an interesting idea. I'd have to think carefully about whether it creates any game-breaking shenanigans.
EDIT: Well, it makes Scout into +4 Cards/+1 Action, so there's that. Similarly, Cartographer and Apothecary both become +5 Cards/+1 Action and Navigator becomes +5 Cards/+$2.
EDIT: Also, if you have a Merchant's Wagon and a Secret Chamber in hand and are hit by an attack, you can draw your whole deck.
EDIT: In addition, you could reveal it during your Clean-up phase if you're putting cards back on your deck (Walled Village, Scheme, Alchemist, Treasury, etc.). That would result in some rules weirdness.