Each landmark becomes a Trial once the game reaches it, and it plays two ways depending on the hat you wear. Same story either side, different reward: Caps get closure, Brims get power.
Worked example: Fallen Fields
On the Cap side you explore the fields, learn the tragedy behind them, and survive the harvester's scythe long enough to realise the grain is hollow, then call a storm down on a field built for nothing but endless growth.
On the Brim side it is a boss fight. You are handed one tool built for the trial, a sword that cannot hurt anything but can parry the scythe once, and you fight the witch's spirit directly. Winning absorbs a piece of their essence as a new spell type tied to their wish: a plant tendril that keys off whatever elemental sigil you cast it with.
Trial bosses feed the same sigil and modifier registry that zone bosses do. They are simply a rarer, mythic tier of the recipes you already know how to build.