Concept
A blank deck of cards that acts as an inventory. In order for an object to be inserted in the card it must interact in a way with the cards.
Ideas
I take a pair of scissors and start cutting a card, only to see the scissor absorbed into the card and transformed into an illustration with a number and suit. I can later take this scissor out of the card.
If I punch a blank card, we can see an illustration of a punch appearing on the card. If it's a full deck of card you wanna insert in a card, you must spring it towards the card.
To produce something like a punch out of a card, you can make it comedic looking by having a full arm come out of the card and punch the individual in front of it.
Questions
How do you insert big objects inside of the card? A person, a car, a house?
How can you insert something object-specific, like two cars crashing, a person lifting an object?
How can you insert a thought or an emotion?
World
In another world, people have no more backpacks, no more luggage. Those who have understood how to take the potential of the blank deck to the highest highs are now seated in the highest positions.
For example, one might end up flying because she has inserted in the card a lot of compressed air and has figured out how to stock large quantities and release it fast and powerful.
Other people have sucked inside of their cards entire worlds and have ultimately committed deck suicide by allowing themselves to be sucked inside of that card. This idea can give birth to a whole corporation that revolves around the idea of allowing people to live young forever by creating Card Worlds where thousands of people have been sucked in and live in communion.
Potential: book, short story, sketch.
What if they would genetically make some bootleg versions and these could be used in the same way but would malfunction from time to time? Or these could be the ones you could use only 1 time whereas the ones you get when you are born can be used for forever.
I would also make it so chosen individuals, those who get Purple, Black, White Backed Cards can use their card in a special way, like @Vasile Bizau said regarding elements combining inside of the cards. This way you can have a Regular Card, a Bootleg one and The Special Ones - and boom, you have the beginning of a lore.
Everyone should also keep in mind that all of these stories can be used as patters for their magic performances. There's nothing better than a nice story to tell your audience, something that they can take with themselves back home and keep thinking about (besides the magic that you are performing).