My suggestion for a gameplay mode it would be a survival mode. In this mode, the game starts at some higher difficulty, like 40 or something, and you dont have to recover the balls, but instead survive while maintaining a certain minimum kill per minute score. Each X time you are able to survive, you get one reward, and the difficulty increase. To win the mode, you have to break the target bellow the Godess, like in normal gameplay. If all of yor dolls die, you lose, and dont win any rewards.
In this gamemode, you could select which weapons you would take. The avaliable weapons to take would be only the ones you already unlocked the correspond card by playing the normal gamemode. Maybe you could also select two cards for the dolls, one for the stats, other for the IA behavior.
There would be two random avaliable stages (like normal mode), each one associated with a type of boost card pack. For each X time you where able to survive, you would be rewarded with one random card contained in such boost. So, if you have to survive for 1 minute and survive for 5 minutes in a stage associated with the game-characters pack, you would get 5 random game-characters cards. This gamemode would not give any bits or money, only the cards.
Playing this gamemode would count normally for the shop cicle. This way, those who want to build a strong team have a gamemode to build those teams without having to rely on RNG; those who want to unlock cards have other way to get cards besides the boost packs (but it could be balanced in such a way that buying the boost packs daily would still be optimal); and those searching for some gameplay variety to recicle they days would have a new gamemode to try out. And this gamemode would not replace the normal gamemode, only add to the game.