So, a while back, I was thinking about Josh Hilden's Shores of the Dead, and half thinking about doing a game based on it. Now, I had a few ideas, but I also wanted to do an honest-to-goodness playing-card based RPG. I thought of the old Dragonlance SAGA system, which was based on a unique set of cards with nine suits and some pretty horrible problems... but, wrapped around it, it's a not-bad system for human-scale conflicts. I tossed it around, and got some downtime while at GenCon to write a bit, and came up with the following.
ODE: The One Deck Engine.
The One Deck Engine can be played with a standard deck of playing cards; contrary to the name, there's nothing preventing you from using multiple decks, either to combine them into one large deck, or with each person using their own deck.
The central mechanic of the ODE is to play a single card, add a relevant attribute and any other modifiers, then compare it to either a static target number, or to an opponent's play. A basic static target number is 12; something that a trained, modestly talented, person will succeed on about half the time.
All play proceeds from the hands of the players and the GM; players have no more than 5 cards in their hand, while GMs have 3 + 1 card per player. At the beginning of a player's turn, they draw until they have 5 cards in their hand; at the beginning and end of the GM's turn, they draw to their hand limit.
Each of the four suits is associated with a different attribute. Clubs are Fighting, Spades are Physical, Diamonds are Reason, and Hearts are Ego (Charisma, Willpower, and the like). Playing a card of the suit appropriate for the attribute confers an additional bonus, called a Trump; when someone plays a trump, they may turn over the top card on the deck and add it to the total. If that card is also trump, they may turn over another card and add it, until they no longer turn over cards of the appropriate suit. In many cases, more than one attribute might be appropriate; someone can win a debate through Reason or Charisma (Ego). A character might avoid a blow because of their finely honed martial arts skills (Fighting) or because they're fast like a freak (Physical), or intimidate someone with brawn (Physical), presence (Ego), or a display of skills (Fighting). If a GM proposes an attribute on a test, a player might suggest a rationale why another might be more appropriate, but a GM should feel perfectly free to say "No, you cannot avoid getting hit because you're so pretty", and thereby use Ego in place of Fighting or Physical). In practice, attributes can be abbreviated with either their suit letter (C, S, D, H) or their attribute name (F, P, R, E).
The cards 2 through 10 add their numeric value to the total, in addition to possible trumps. Aces add only 1 AND represent a complication if that Ace is not Trump. Face cards may be played with any other card, and have two effects. First, they allow the player or GM to draw up to two cards (subject to the hand limit). Secondly, if either the face card OR the number card is trump, then they may claim a trump draw; on a Physical action, both 2H QS and 2S and QH will be counted as a trump. The card rules apply even if the card appears in a trump draw; if the aforementioned Physical action turns up the AH, then there is a complication, even if the total is high enough to succeed.
Jokers are optional; they are not necessary to include in the deck though, if someone is playing with Jokers, everyone should be. A Joker is an auto-success on an action; play or trump draw a joker, and the action succeeds as if the total were 10 higher than the difficulty, no matter how high the difficulty. However, once played, a Joker is removed from play.
To the total may be added modifiers, either because of good tools or to represent relative differences of tools or circumstances; attempting to race a tuned Porsche in a Prius is going to result in some modifiers, because no matter how good the character is, the Porsche has some obvious advantages. On the other hand, even to someone not trained in research, having a library (or even Google) on hand will provide a bonus over trying to figure everything out in their own head. As a general rule, modifiers should not exceed +/-5.
Health and Damage
A character's health is equal to the sum total of their attributes (though a GM might grant extra health to important or large characters, or remove it from particularly feeble or unimportant characters). Each point of damage taken reduces one attribute (of the target's choice) by one point; reaching 0 in a single attribute due to damage will knock someone unconscious; reaching 0 in all attributes will result in death. Reason and Ego damage heals at the rate of 1 point per hour; Fighting and Physical damage heals at a rate of 1 point per day.
Succeeding in an action to damage someone causes 1 point of damage. Every 3 full points by which the attacker's total exceeds the defender's total grants an additional point of damage. As such, playing a Joker (exceeding the difficulty by 10) will result in 4 points of damage; 1 point for succeeding, and 3 points for beating the difficulty by 9.
It should be noted that such damage does not only need to result from physical conflict, nor does physical conflict always have to result in damage to Fighting or Physical attributes; a debate might slowly chip away at someone's mental and physical reserves, or a painful cut might not inhibit fighting, but certainly damage concentration (reducing Reason or Ego). Also, many conflicts will not be to the death or even unconsciousness; not everyone fights to the death.
Characters are simply defined; they have 4 attributes and 3 skills, plus anything the GM or player deems relevant. While attributes can be determined in any number of ways (random plays from the deck, for example), the simplest is to start each attribute at 3, and divide 8-12 points between the four, as desired. A human average is 5; someone with a 5 is about as strong, agile, smart, and personable as you'd expect, whereas someone with a 3 is obviously somewhat disadvantaged in that attribute, and someone with a 7 is obviously gifted. A usual maximum is 10, but a GM might allow higher numbers in rare circumstances or if using exotic races (like minotaurs or aliens).
Skills should be specific while leaving some room for nuance; there should be plausible reasons why a skill can't be used in every situation. One sniper character might choose the skill of Rifles; another might choose Sniping. The first wouldn't get his bonuses with a scoped pistol; the second would be of less use in a stand-up fight. Skills may have certain attributes which they suggest, but they're not tied to a given attribute; someone with skill in Rifles would frequently apply it to their Fighting skill, but might also use it with Reason to repair or upgrade their weapon, or with Ego to strike up a conversation with a gun enthusiast. When a skill is applied, there are two effects. First, there is a +2 to the test's total. Secondly, Aces are only complications if the wrong color; they don't trump any more often, but an Ace of Hearts on a Reason action is only a 1, not a complication. If two skills apply, the character only gets the benefit of one.
At certain times, depending on the GM, characters may improve. The GM may choose for characters to learn an additional skill, or may give characters the chance to improve an attribute. In order to improve an attribute, the player declares which attribute they wish to improve (or the GM may have decided for them), shuffles the deck, and draws a single card. If that card exceeds the current value of the attribute, the attribute improves by 1. Trumps do apply to this draw, including face card trumps, but non-trump Aces reduce their related attribute by 1.
Attempting to strike a person: Fighting v. Fighting or Physical
Having a Debate: Reason or Ego v. Reason or Ego
Jumping a chasm: Physical
Driving a car dangerously through heavy traffic: Physical
Having a car chase: Physical v. Physical
Researching a problem in a library: Reason
Asking about something in a tavern: Ego
Making Plutonium out of common household items: Reason
Determine if someone is lying: Ego
Throwing darts in a pub: Physical or Fighting
Minutely reproducing the hang gestures you used for years on a TV show: Physical or Ego
Remembering details: Reason
Skiing the K-12: Physical
Figure out how to lock a blast door: Reason
Remember: Players are free to suggest other checks if they can come up with a reasonable rationale. GMs are free to say no if they think the players are being a bit stupid.