I've been mulling over a CCG for a while now that would be time-travel based, but I haven't been too sure how to go about this. The inspiration for this was the temporal cold war from Star Trek: Enterprise. You don't get to see to much of it in the show, but the concept of different times using temporal warfare against each other intrigued me.
As to the battlefield, I figured a hodge-podge of different timelines would be a good place to start. All these time-travel shenanigans would have reduced history to a wasteland battleground with samples from all eras. Time travel as we think of it would have become irrelevant since the timeline has been messed with so much. (Muddying the timeline like this would also prevent boring moves like "I go back in time and kill you when you were a kid!" scenarios.)
With direct time travel more-or-less out of the picture, I figured it would be interesting to explore how the pasts of characters, events, and locations could be changed to "edit" the present version. Alternate versions of things and people would be common, and could be messed with by finding the right way to alter their pasts. Characters can be "edited" to have different eras intersect with their histories; samurais with access to semi-automatics, or scientists able to tame dinosaurs. Everyone knows something is off with the timeline and that things don't quite make sense. In terms of a CCG, this could either be done as pre-packaged cards ("I play "Uzi Samurai"), cards with alternate versions that need to be activated (a bit like the transform mechanic for Magic werewolves, but more possibilities would be preferable), or activations through other cards ("I use my temporal agent to tweak the timeline and give my samurai +3/+0 and range").
Now onto the mechanics:
With access to all of history, the different "factions" (or MTG-like colors) would be different eras in history. One way to represent this would be by having cards come from different eras, numbered from 1 to 9 (2nd era, 3rd era, and so on). Early eras would have cards like dinosaurs, cavemen, and a Wrath of God-style meteor strike. Then we move on to antiquity, feudal Japan, the middle ages, the Age of Discovery, the Industrial Revolution, the present day, and the future before time travel was invented. (As soon as time travel was invented, the eventual damage to the timeline happened instantly (since we are dealing with time here) and for all intents and purposes there is no future beyond that.) Different ways to mess with this would be:
- Each player has an era number. This would start on 1 or 10, representing their starting era (yes, players can start in the first era since all time-travel is rerouted through the Big Bang! Duh). If a player wants to play a card from a different era, they have to pay the difference between their current era and the era of that card, since the further up- or down-stream you travel, the harder it is. So from starting era 1, playing a Panzer division from the 7th era would be more expensive than playing a Roman legion from the 4th. Once that card is played, though, the player's era becomes the era of that card (represented by a D10) and era differences are adjusted accordingly.
- Both players share an era number. Since the battlefield is an erratic mess of timelines, a D10 is rolled at the start of every turn (players take shared turns), and the result is the current era. On a 9 roll, my Panzer division comes into play cheap, but your Roman legion will have to wait until next turn since you can't afford it.
- Use Planechase. Similar to the above idea, but instead of rolling a D10 players bring 5 era cards each to the table. They are shuffled together at the beginning of the game and an era is revealed from the top of the deck at the beginning of every turn. This allows for additional effects to be included on the cards, as well as allowing players to steer the flow of the game some more. I can, however, imagine turns see-sawing, in that one turn favors player A and shuts down player B, and the next turn does the opposite.
- Use suspend. This Magic mechanic would be useful in creating a cost for cards, in that pulling an object to you from further in time would take more of a delay.
- Use number loyalty. I'm not too sure this idea works well, but having every card have two era numbers (one for the aforementioned samurai, another for the bazooka he wields) would allow a player to "jump through time": say the samurai's eras are 3 and 8. My first play of the game is "free", and I use it to play him. These are my current eras; I can only play cards that have either a 3 or an 8. This lets me play my Knight-hijacked Panzer division, which boasts an 8 and a 5. These are my new numbers. And so on...
Anyway, these are simply ideas. I don't really know where I'm going with this yet, but a shape is starting to emerge. Please feel free to comment on these ideas or add more!