I don't think I've seen this particular idea proposed, although it resembles some other ideas as well as borrowing from similar concepts in CCGs. The Environment cards (I don't like the name, but we'll start with it) are, like Knights, a stack of 10 unique cards with different names and effects, and their order would be randomised at the start of a game. Environment cards would have a few things in common:
1. Rather than being cards you add to your deck, they go straight into play when gained and don't get discarded from play.
2. Only one Environment card can be in play at a time.
3. All Environment card have a "while this is in play" effect, usually affecting everyone.
So, where not specified, Environment cards have the sole type "Environment" and card text that includes "When you gain this, put it into play. Do not discard this card during Clean-up. When another Environment card comes into play, trash this card." I haven't worked out how to avoid the issues caused by cards like Watchtower other than to rule that they can't target Environment cards.
The Environment cards could include (costs not listed because I don't know how much they should cost, and names definitely open to alternatives):
Free Market
While this is in play, at the start of each player's turn (including yours), he gets +1 Buy. When you buy this, +$1 +1 Buy.
Metropolis
While this is in play, cards cost $1 less, but not less than $0.
Co-operative
While this is in play, at the start of each player's turn (including yours), he gets +1 Action. At the start of your turn, +1 Card.
Inflation
While this is in play, Copper produces $1 extra when played.
Mulligan
While this is in play, at the start of each player's turn (including yours), he may discard his hand. If he does, he gets +4 Cards.
Rules clarification: You may choose to resolve this along with other start-of-turn effects like Duration cards in any order, but you must resolve the whole discard-and-draw effect before resolving any other effects.