This is an article of general advice for designing individual cards and other game objects.
These are effects that only work if a condition is met.
The Radlands expansion camp "Battering Ram" only works if you have six people.
There's a huge amount of design space and depth in conditional effects. Just pair any condition with any effect. They're relatively elegant, and tie to other actions in a little chain, as the player tries to create the condition.
When using conditional effects, the condition must actually be something that the player can control. If it's something the game controls, then it's essentially a complex way of doing randomness.
There's little difference between "gain 6 dollars when your roll for the turn is 6" and "gain a dollar each turn". Another example is a magic amulet that gives you a bonus whenever you defeat a goblin, but you have no control over whether you'll be fighting goblins or not.
The Radlands expansion card Death Ray used to require that you pay six water to fire it. This was a huge cost, for a huge effect. You could get six water at once, by discarding several cards with the water icon in their corner. However, you didn't actually have any control over what cards you drew. Only a few had the water icon. That was an uncontrollable condition. Later, I changed the cost to just discarding any five cards.
Even weirder are uncontrollable conditions, that the opponent controls. The opponent will not just blithely meet the condition for you. This often just stops the opponent from doing the thing.
Several playtesters and I all invented the same camp for the Radlands expansion. It was a "super-hospital" that would restore (heal) exactly two of your injured people. In practice, however, the opponent just never left two of your people injured. They'd injure one, and only attack the other one if they could kill it completely. That outcome was very different from the intended effect of the super-hospital.
If a player doesn't have enough control over a condition, you can give the card a second effect that helps create the condition.
The Radlands card Zeto Khan lets your event cards happen instantaneously, rather than being delayed. This is fun, but what if you don't draw any event cards? I gave him a second ability, that lets you draw three cards then discard three. This gives you a very high chance of drawing event cards.
I consider the best category of designs to be the rule-changers. These cards simply make something, or the game, work differently. These can't be easily found, like the other types do. They tend to be simpler, more unique, and more thematic. Try to make more.
The new Obelisk camp from Radlands simply says "When the last card is drawn from the deck, you win."
A good way to add a lot of strategy to cards is to create a downside, or undesirable consequence. Make cards that also affect (negatively) your own stuff. This can also be an opportunity for cleaning up the game state. The player can discard useless cards or resources.
Avoid permanent downsides. They're almost always un-fun, no matter how interesting or fair they are. Permanent benefits are great, but keep the downsides to being once-off losses.
Let the card do one of two (or more) things. If you have effects that are cool, but often useless, combine them into one card.
The old Magic card "Shatter" would destroy an artifact card. It was necessary for that effect to exist. However, if the opponent didn't have any artifacts, Shatter was useless. These days, they make cards like Abrade instead. It lets you destroy an artifact, OR deal some damage to a creature. This means it's always useful.
Go through all the rules of the game, and make cards that break them.
Many cards have an ongoing effect, and are thus useful at the start of the game, but become useless later on, as you'll get less time to use them. If your game has a "points" system, give cards with an ongoing effect a high point value. This means players will want to play those cards at the start of the game, for their effect, but also at the end of the game, for their points.
Give the player a way to use unwanted cards. This also lets you give out more cards than the player needs, which increases choice.
In Radlands, cards can be "junked". There's an icon in the corner. You discard the card, and do whatever that icon says, rather than actually using the card.
In my gangster game, to play a card, you had to also discard some number of other cards from your hand.
In another of my prototypes, a few actions simply require that you discard one card. It's not much, but it makes every card useful.
Every card should relate to its context. With some situations or other cards, it's more (or less) useful. The player should have to evaluate the card, each time they draw it.
In Radlands, you choose three Camp cards. They do something, and also give you a few cards for your openinng hand. Obelisk doesn't do anything, but gives you an extraordinary three cards. Obelisk did have a downside, though. It began the game already damaged. It was a strong camp, which was fine, but it had no context. It didn't matter what other camps you had. You always took the Obelisk. It was still a fun camp though, so I changed it to destroy itself. Now, it's a weak camp, but an amusing curiosity that you need to evaluate each time you draw it.
There should be no cards you always (or never) play.
However, the player shouldn't just be able to reduce this analysis down to a very simple formula.
My gangster game had a Machine Gun card. It cost some money to play, but would increase all your future damage by 1. What's the strategic analysis created by the Machine Gun? It boils down to whether the game is going to end soon, or go on much longer. If the game is about to end, you don't play this card. If the game will go on for a while, you do play this card, as it's a worthwhile investment. This card's strategic analysis is too easily resolved to a simple question, and the card was culled from the game. (If every card was long-term like this, this card would be appropriate for the game, as you'd have to compare them all with each other. That would be an interesting choice.)
Choices that simply resolve to "am I winning or losing" should also be culled, for the same reason.
If a choice can be made now or later, both should be viable choices. There should be no card that you always play immediately. There's a lot of strategic depth in choosing timing, without added complexity.
My gangster game also had a Tank card. This card was like the Machine Gun card, but it instead doubled the amount of damage the player did. I made Tank work, by making it destructible. It had its own Health tokens, and protected the player's health. The Tank was discarded when it ran out of Health. This new Tank now had a second purpose — protecting the player, but the card's main ability now required much more strategy. You needed to choose when to play the Tank, because if you played it in the wrong context, other players would pummel you before your next turn, destroying your Tank before you got to use its double damage.
My gangster game also had a "you've found some money" card. You played it, and you got a dollar. There was never any reason to hold this card. You'd always play it, and always play it immediately. The card it became was Kidney Donation. It gained you two dollars, but also cost you two health. This card was super-simple, strategically interesting, was played only some of the time, was usually not played immediately, and was also humorous and relatable.