D&D has a mechanic called inspiration, which is a single point that can be used by the players to gain advantage on a given die roll. That mechanic is so weak that many tables ignore it.

The Conan 2d20 system has an interesting mechanic called momentum and doom, which are points you can use to accomplish things or improve results and points the game master can use to make encounters and predicaments more challenging. That particular system is too deeply integrated into the general mechanics of the game to be portable, but I liked the idea of it enough to try to create something more system-neutral.

My luck and danger mechanic has some similarities with both.

Luck

You can spend a point of luck to reroll a die roll and keep the higher of the two, but luck can only be used once for a given roll. You can also use luck to power certain class features. Luck cannot be used to reroll the results of carousing.

You can earn luck by carousing and by good role-playing. I also have a “hero die” that grows as you advance in level and adds to your damage; rolling a 3 or higher gains you a point of luck–this is copped from the gonzo old-school system Dungeon Crawl Classics.

Luck cannot be hoarded indefinitely. It resets at the end of the adventuring day. But any luck you gain by carousing after the end of the adventuring day you keep for the next adventuring day.

Danger

Danger works for NPCs and monsters just like luck: re-rolling die rolls and powering special features. But the GM can also spend a point of danger to generate complications when the heroes are doing too well.

• A hero trying to be stealthy near some bushes disturbs a pair of birds, who burst out in a flutter.
• A guard unexpectedly turns up.
• The heroes encounter a wandering monster or a dangerous (but not necessarily hostile) beast.
• Something the heroes are relying on to be solid starts to crumble.
• One of the heroes’ supplies runs low because some have been broken or lost.
• A thing the heroes knew might happen at some point happens.

The GM gains one point of danger anytime a player rolls a natural 1 on a d20 (attacks, skill checks, ability checks, including when it is canceled by a recovery or luck reroll).

Also, a given location can have danger associated with it, and a monster or NPC can enter an encounter with danger, sometimes 2d4 points. Whenever this occurs, you as the GM should foreshadow the danger with an ominous sign.

• A warning from an wizened old woman
• A door flung open or slammed closed by a gust of wind
• The flurry of a cauldron of bats or conspiracy of ravens
• A murder of crows picking at a carcass
• The howl of a wolf or roar of a monster in the distance
• Dark chanting carried on the wind from the distant villains
• A spooked horse and the flash of a falling star
• A lightning crack or sudden thunderstorm
• Sudden stillness of wind and birdsong
• A tombstone with the same name as one of the heroes
• Strange and ominous noises from the underground passages beyond

You should explicitly say this is an ominous sign portending danger.

Danger is reset to zero only when the heroes resolve the conflict of the adventure and return home.