But there are numerous ways in which cards can be incorporated into a game design.
The simplest option for using cards in a game is probably substituting them dor the dice as a variant form of randomiser. Standard playing cards are great for this, because they add extra degrees of information that might be quickly used to add flavour to result outcomes using suits and ranks. Tarot cards might be even more useful because you gain access to major and minor arcana, interpretations of cards in upright and reversed configurations, as well as thesuit and rank values. Then you could always play with the option of developing your own custom deck to specifically explore the thematic content of your world, but this is probably the most time consuming option.
A different option for regular cards can come through character generation. I've played with this in my "Hold Em NPC Generator" (which happens to be on sale at the moment), and I've seen a few other games play such as Through the Breach with the concept as well. In most cases ou lay out aome cards in the form of a tarot spread to get some tools to continue the character building process.
But in this example, I'm thinking of unique cards, each containing an ability, power, or special item for easy access during play. A fairly minimalist character sheet, a fairly streamlined set of rules, and any complications are specifically written on cards, and only available to the characters who have those cards associated with them. I'm almost thinking of the way that "Powered by the Apocalypse" games open up new avenues of play to specific players when their characters purchase specific moves, but also echoing back to the thoughts I had years ago when I was developing my vector theory of game design. The cards will provide options for manipulating the narrative, and might work to change the pacing of the story (speeding it up or slowing it down) or its direction (social conflict to physical conflict to metaphysical risk to anything else that might be important for the stories we want to tell). Only through massive degrees of success will a single card allow both pacing and directional changes.
Since I'm specifically thinking about a game that is inherently magical, these cards will have a range of effects. The basic effects will induce minor changes within the rules of the game, more powerful effects will induce more significant changes to the narrative within those rules. The next step (which may involve magical influence) will not manipulate things within the rules of the narrative, but may very wellhange the rules themselves to a minor degree. The most powerful effects basically take the narrative duties and the control of the world out of the hands of the GM and into the hands of the player within a limited scope (where such a scope might be limited in duration, physical range, sphere of metaphysical influence, some other limitation, or a combination thereof).
So, a combat card might offer: minor damage > major damage > minor damage with an immediate follow up action or subtle transformation of the scene > complete control of the conflict situation. An investigation card might offer: trivial information > minor information that's useful > information from a different perspective that you didn't expect at all > complete information regarding the sphere of questioning.
This is basically the way I'm seeing spells working in the game, but all abilities are spells and all effects are the result of the characters imposing their will on the reality around them. Each card reflects a different way of imposing that will, and a larger collection of cards creates a more versatile repetoire of reality manipulation.
Each character will be defined by a simple pocketmod booklet for their "character sheet", where starting characters might begin with a standard set of three or four cards appropriate to the type of stories the group intends to tell (combat, investigation, social intrigue, exploration, athletic feats, etc.), then each character might have a choice of two cards from their occupation, upbringing, genetic heritage, or some other characteristic, so different members of the group can fill specific niches in the narrative. Everyone can always do something, the cards simply offer new options for things to do, and the character's statistics provide an indication of how well they can do it.