In general, recent RPGs have some kind of "X system", where X is some entity or concept that, flavor-wise, drives the Game System. This trope describes what happens when X is also a key plot point. Those things you have to find all of? Maybe there's a special one that could save or destroy the world. Maybe each time you get a Plot Coupon, it comes with some new spells, abilities, or commands you can use. The point is, a single concept is central to both the gameplay or customization and the narrative at hand. Contrast the MacGuffin, which serves no purpose besides driving the plot.