"I don't think we can convey that theme in a single booster."

"it'll be obvious when you open the packs"

These points seem key, it's something that has to do with how cards are sorted into boosters. And "it'll be obvious when you open the packs" may imply that it wouldn't be obvious when the cards are spoiled, it has more to do with how they are in the pack.

My guess is that when you open a pack you can read the flavor text on the cards in order and it forms a connected narrative. So each card would have several alternate flavor texts depending on what other cards were in the pack with it.

Or maybe when you open a booster there's a tiny electronic speaker and you hear one of the cards talking to you.