I think this is definitely a good idea and would make it a lot more like a RL card game. It would also be a good way to introduce new cards. Imagine the surprise when you buy a booster pack, only to be faced with a completely new card - even if it's not a great card, it's still something nice to have.
How would they work? Would you be able to buy all entropy packs? Or would it be packs able to contain any element? Or perhaps booster packs containing a possible 2 types of elements which pair up (Aether/Time, Life/Death, Fire/Water, etc)?
I think if we went for packs made of just one element, you should only receive a small number of cards. It's pretty obvious what you might, so with such a low stock of cards, you should get a low amount in the pack. For packs containing any card, then these should be large packs (large array of cards to get, so a large pack to increase chances of something you want). Then follow the same rules for the 2 element packs, medium array of cards, medium pack size.