Right, here's my take on a time-travel TCG. I felt the best thing was to have a nice compact rule set that laid foundations for wide design space without being overly restrictive, and that made sure smooth gameplay was always available to every player via a strong, open-ended resource system, fluidity of card function, and cards that helped each other work rather than requiring each other to work. It had to retain the flavour of combat - which a TCG really does seem to need to be successful - while working it into a time travel system that was provocative, interesting, and made sense. Basically, it had to be enough like other games to be easily picked up, and different enough to be appreciated. The following is a very rough mockup that I basically wrote offhand, and I'm sure it needs a lot of work - but I also feel that something very like this is our best bet. Thanks for your constructive criticism and advice.
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TimeStrike
Introduction
In TimeStrike TCG, you and your opponent(s) represent time-travelling heroes fighting a contest of wills throughout the ages. TimeStrike is based on the fundamental concept that only a time traveler can take anything forward in time, but nothing backwards. Hence, nothing can exist in a time before it was invented - this also explains why Rome was never conquered by Panzers, but Mexico has a black-ops team composed entirely of Mayan warriors. This principle also compensates for the fact that future technologies will always be superior to those of the past - peoples from the past will always be fighting on their own terms, but future units will never be able to attack the past with anything more than the current level of weaponry.
Card Types:
Units - these represent people.
All Units have a resource cost, a faction imprint and home time period, a Combat number, an Influence number, and a Stealth number.
Equipment - these represent things your units are able to use.
All Equipment have a resource cost and a faction imprint and home time period. They typically have numbers denoting the bonuses they give to Combat, Influence and/or Stealth.
Events - these represent actions that your units are able to do or take advantage of, including those brought from other eras.
All Events have a resource cost and a faction imprint and home time period.
Keypoints - these represent historical places at times with great historial significance, which serve as branching points for many possible futures.
Keypoints only have effects and a time period.
Time Periods
There are ten time periods to which a card can belong:
1 - Prehistory (~3000 BC backward)
2 - Antiquity (~3000 BC to 300 AD)
3 - Medieval (~300 AD to 1300 AD)
4 - Renaissance (~1300 AD to 1600 AD)
5 - Colonial (~1600 AD to 1900 AD)
6 - Industrial (~1900 Ad to 1945 AD)
7 - Nuclear (~1945 AD to 1991 AD)
8 - Cyber (~1991 AD to 2050 AD)
9 - Space (~2050 Ad to 2100 AD)
0 - Apocalypse (~2100 AD forward)
Zones
Past - this represents things that no longer exist (aka graveyard or discard pile)
Present - this represents things that now exist and are in use (aka battlefield or play)
Future - this represent things that could exist in the future (aka library or deck)
Plan - this represents things that exist, but aren't being used yet (aka hand)
Factions
Factions would include groups such as the Roman Empire, the British, the Chinese Dynasties and so forth. They would have different play styles based on their respective philosophies and historical characteristics.
Beginning of Time
Each player has a fifty-card deck and ten Keypoints. Each player sets aside ten Keypoints numbered 1 through 0 (one of each era), face down and in chronological order. He or she then draws five cards from their future into their plan. The oldest player goes first and play rotates clockwise.
Round Structure
1) Time Phase:
The time period begins. Each player flips up their next Keypoint such that it overlaps the previous one's picture but leaves its rules text visible. If the previous Keypoint was Apocalypse, start over at Prehistory as civilization begins again.
2) Restock Phase:
Each player loses all previous resources and gets a number of resources equal to the number of that keypoint. (e.g. during the Medieval Era, each person gets three resources, using tokens to represent them. Medieval cards will rarely cost more than three resources.)
Players can move resources between any of their Units or Equipment with the same faction imprint.
3) Draw Phase:
Each player draws a card.
4) Play Phase:
Each player (in rotation order) gets to play Units and Equipment using the resources they have by putting the card from their Plan into the Present and moving that many resources onto it. (e.g. a Breton Knight costing 3 would enter the present with three resources on it.) Resources can be moved from the player or from other cards of the same faction imprint. When a card has no resources on it, it's sent to the past. (e.g. The Breton Knight could also be equipped with a Longsword if the player moved a card from the Armorican Warrior they played last turn and sent it to the Past.) This effect streamlines deckbuilding decisions and prevents old units from becoming useless later on.
Equipment comes into the present equipping a Unit if possible. It can be equipped to a new unit once per turn. When a unit dies, Equipment falls off and remains in the Present.
Many cards have variable costs (e.g. a Peasant Army might get stronger the more resources invested in it). This also lets cheap cards be useful when drawn later in the game. Resources on a card are used to pay the costs of its abilities.
Players can play Events or card abilities at any time specified by paying their resource costs. Events can also be paid with resources from cards of the same faction imprint, but abilities can only be paid with the resources on the card with the ability. (However, resources can be distributed between those cards during the Restock phase.) These resources are lost. Some events and abilities also have a variable cost, and others are free (particularly those with symmetrical effects or that wouldn't typically require resources in real life).
Some plans take years to come into fruition and some technologies take decades to perfect. These are represented by playing setting cards with the Long-Term keyword aside face down and paying a portion of their total resource cost each turn. The card is played when it has enough resources on it to be played normally.
A player can play a Keypoint in their hand with the same time period as their current one and replace it. The old one is sent to the Past.
Cards can't be played before their home time period.
Players may find themselves with extra resources, in which case two resources can be spent to draw an additional card, once per turn.
5) Attack Phase:
Each player (in rotation order, with any number of rotations) can declare an attack for one of their units. Units can attack another unit, a player or a Long-Term card.
If a Unit is attacked and not intercepted, both players have a chance to play Events and abilities. Then the Combat scores of those units are compared and the loser loses a resource. If there's a tie, both are sent to the past.
If a player's attacked and not intercepted, they put cards from their Future into their Past equal to the attacker's Influence.
If a Long-Term card is attacked and not intercepted, it is sabotaged and loses a resource.
A unit with equal or greater Stealth than any attacking Unit can intercept it and force it to battle itself instead of its original target.
The Attack Phase ends when a full rotation is completed and no player wishes to make a further attack. (Every unit gets a chance to attack, but players must prioritize the order in case weaker units are killed.) Units can attack only once each round.
4) Play Phase 2:
Each player (in rotation order) can play further Units, Equipment, and Events.
5) End Phase:
The time period ends.
End of Time
When a player has no Future, they are dead.
