Okay, so Andrew Shields over at Fictive Fantasies has started a cool new project called the Caezarette Cypher Deck. He's set himself an ambitious goal of 101 cards--but then each card just needs a name and small painting at a minimum. As I commented to him, I've always been interested in magical card decks, whether the tarot or Deck of Many Things or Pathfinder's Harrow deck. I find that although I enjoy the cards by themselves they are difficult to work into actual play at the table. Either a deck is too powerful--essentially a set of magic items, or it's effects are so random and dangerous that the players just chuck the damned thing in a well, or it is treated as a single minor item which simply gives bonuses to divination or whatever.
Maybe a better approach (which I've not actually tried, heh) is to make each card a separate very minor magic item usable by itself. For instance in this basic mode the tarot Three of Swords could provide +3 to an attack/damage type spell (or penalize defense against such a spell by -3). You could restrict use of this mode to perhaps once per day or something. Then on top of that you get bonuses once you've collected a whole suit, such as all cards can now be used in the basic bonus mode twice a day and bonuses are increased by 50%. Then there would be a third tier for collecting the entire deck. I've never liked the approach where you draw a random card and have some random nonsense happen. I want for a player (or NPC) to be able to eventually become a skilled user of a deck.
To add variety, you could also divide decks into types based on their attunement. A deity of luck/chance could provide divination decks to clerics, prophets, oracles, etc. Wizardy types could build arcane decks as a spellcasting focus attuned to their mana. You could have restrictions bonding a deck to just one person and staying bonded as long as they are alive. Another idea is to have the deck itself be non-magical, but usable as an expendable spell component. In this case you might have the suits in the deck correspond to schools of magic, such as fire or illusion. When casting an illusion spell the caster could use a card from the illusion suit to boost the spell's power or give it an additional twist.
For the campaign world I'm building I see decks of this type as mostly made by either clerics of deities whose portfolio includes luck, fate, or change or by certain arcane magic users seeking an aid to divination. I'd also like to have equivalents for non-literate (barbarian) cultures and for nature-attuned types like druids.