So I've been working on this game / game system for many years off and on. This isn't the most complete version of it that I've had along the way, but it's what I've got now (I've thrown out a number of ideas / subsystems in the hope of simplification).

The Core (stuff that I don't want to change)

Alongside each deck are a small number of cards that I call Secrets (name subject to change). These are relatively powerful cards with ongoing effects, and you don't even have to be lucky enough to draw them from your deck! However, they also keep track of your opponent's score, so the more of them you have active or in play, the closer your opponent is to winning! You start with them all face down, and each time your opponent "completes a Quest," you flip one of them over and its game text goes into effect. When you have no more to flip over and your opponent completes another Quest, you lose.

Like most CCGs, cards in your deck have a number on them that indicates their cost, with a higher number for more powerful cards. I call it the card's Fortune Number. However, in this game, the Fortune Number doesn't restrict when you are allowed to play the card. Instead, it's used as a random number generator whenever a card needs to have a random chance of doing something or not: you reveal the top card from your deck and compare its Fortune to a given threshold. This is called Scouting. If your Scouting result is lower than the threshold, something (good) succeeds or something bad is prevented. Opposite if your Scouting result is greater than or equal to the threshold.

At the end of every turn, you can (without needing any card to allow it) Scout to complete a Quest. The threshold number you need to beat to complete this Quest is the number of cards placed in your Invested Pile. Lots of cards let you place other cards in your Invested Pile (with restrictions, such as once per turn). Then once you complete a Quest, of course your Invested Pile resets to zero cards.

So the game is a race to fill your Invested Pile faster than your opponent (and/or have lower Fortune cards in your deck than your opponent) so that you can complete Quests faster than them -- but each time you complete a Quest, your opponent gets to activate a Secret card to help them fight back.