This is to you:
Firstly, thank you for making an awesome game! If I wasn't a broke fool I'd be buying shards and monsters like crazy! I love this kind of stuff. Designing Worlds for other people to play...it's a dream come true honestly. (in a very minor form)
Secondly, I think you could add more depth to the game by adding:
Each of them range from certain sizes/costs/perks depending on their rarity. Small 2x2 Huts for swamps or Crypts for the graveyards, all the way up to 8x8 Pyramids for the Desert and so on. Maybe even have them path-able so players and mobs can enter them?
Plus adding more Heroes: (These are just some ideas)
(possibly eventually separating destructible and non-destructible into separate categories, and turning one, or both into sets of cards themselves; that are generally much cheaper to craft than other things)
I think making it multiplayer would be a bit of a stretch on the overall stability of the game but if you could somehow manage to allow people to join a party and enter a zone together, then you got a perfect recipe for:
After that you could expand it into PvP, including simple Death Matches and 1v1's, too objectives such as Capture the Flag or Search and Destroy. (Maybe even try delving into MOBA style gameplay with Automatic Mob Spawners and Tower Structures?) All of these ideas could reward players with Glory or other types of currency, making up for dramatic increase of cards that would be getting added to the pool.
OH ONE LAST THING! Stackable Foundations (within a height limit of course) and sloped foundations. (works the same as stairs, adds more quantity, and it adds a much needed aesthetic variety!)
I know it's all a stretch and it all takes time, but what do you think Players, Creators?
I'd be the first to volunteer to help if I was at all qualified.