Let's have some improvements that mess with the economy! Since card price depends on the number of cards owned by players, here are my ideas.
Underground Storage: Level 1: Store 20 common cards. Level 2: Store 35 common cards & 10 uncommon cards. Level 3: Store 50 common cards, 20 uncommon, and 10 rare cards.
These cards are put up into the storage. No player owns them, they are stored to either rise the price or even empty the market, leading to not being able to buy certain cards. Now, we need a balance to this.
Black Market: Level 1: gain access to discounts & specials on your cards. Level 2: Yours and one other element of your choice. Level 3: yours and 3 of your choice.
You can make the cards available change every 3 days or so. Put up like 2 cards from each element at random for some awesome price, able to be bought no matter the number available using normal means. You can have like a 70% chance for commons, 15% for uncommons, 10% for rares and 5% for nymphs, shards, other fun things. (limit like buy 1 of the super rares per time they appear.)
Other ideas:
Arena: Allows your city to be a possibility for the circus to stop. Passively each level increases your's people's happiness (increasing money gained or something.)
Portal: Level 1, 2, building. Level 3, travel to targeted capital. Targeted capital must have a portal targeted to your capital. Travel takes 1 day.
Tavern: Allows bets to me made towards the lottery. A bet made in the city grants your city revenue. The lottery can be an awesome bonus (+XP, cool solo quest chain, cards, electrum, super rare card) Level 1-3 decreases the cost of 1 ticket.
Builder's Guild: Decreases the cost and resources for other improvements, like 5% per level.
Assassin's Guild: Increases your element's ability to put warrants up on people. Perhaps decreased cost to post, de-buff the target, hide your element when you post.
Temple: Spreads your element's influence. Level 1: Hexes surrounding your capital take 4 actions to neutralize (able to throw 2 banners on the hex) Level 2: Same can be done to hexes surrounding your towns. Level 3: can be done to hexes 2 hexes away from your capital and towns.
Guild Hall: Access to quests. Level 1-3: easy, medium, hard.
Library: Access to information that could make quests easier, faster. Could be boring though...
Might think of more later...