If I understand the OP correctly, you are looking for a mechanism that measures the players' actual impact on the state of the in game fiction?
To that end, I would use some sort of tangible Reputation mechanism. Using 5E just as an example, i would tie it to CR because the more challenging a situation the PCs overcome, the more impressive the potential reputation boost. It would probably be relative to party level, though, so PCs that consistently aimed for low threat challenges would see their Reputation go stagnant or even decrease. In addition to a numerical value, Reputation would also have a "judgmental" value related to broadly defined, setting appropriate ethical and moral aspects. Yes, the PCs stopped the bandit raids, but did they do so by taking control of the bandits, by murdering them all, by convincing them to change their ways? Etc.
Reputation would have a couple impacts in play. First, your Reputation would serve as a "gate" for certain kinds of access. Want to talk to the King? You need to be this tall (Reputation wise). It would also impact things like reaction checks and other interaction rolls, with that "judgmental" aspect determining exactly how on a case by case basis.
Finally, I would let players burn Reputation to gain aid from appropriate NPCs and/or factions. Because they needed help, tho, Reputation used this way is lost. Points spent could be used for information, helpful items or even accompanying NPCs or henchmen.
Something like that anyway.