Years ago, while reading Eiji Aonuma talking about how he wanted to shake up the Zelda formula, especially by allowing the player to do the dungeons in any order (which gaves us « A Link between Worlds » and « Breath of the Wild »), I had an idea for a central mechanic following this intention.
The vessel for this mechanic is a sentient lock named Talock . He was the one keeping close a chest (sentient too), where monsters where sealed but someone opened it. So he latched onto Link’s arm and refuse to leave until monsters are sealed again.
He comes with a handy set of base abilities (shield, grapple, hint system,…) from the beginning, similar to the Sheikah Slate in Breath of the Wild.
But his main feature is the ability to turn monsters into items, allowing to expand the base toolbox of the player just by placing specific enemies in desired locations.
Items can be tools to resolve puzzles or just be weapons for fights. They have durability/limited quantity per absorption but item types summoned can be upgraded by absorbing corresponding elite (golden) enemies scattered through the world. Bosses turn into armor sets (no durability since you can only beat them once) with special properties/abilities.
Due to its connection with monsters, the lock can also enchant monsters to make them allies, allowing more potential systemic fun (infiltrating a bokoblin camp with an enchanted bokoblin for example) and monster mounts (horses are too mainstream!).