Key rings and tool belts

The mark of a good Metroidvania is that new items and abilities feel like tools in a belt, not keys in a ring.

There’s nothing wrong with, say, DOOM having the player collect red, blue, and yellow key cards - but that’s just scaffolding. It’s just to provide pacing and a series of objectives. It’s not the heart of the gameplay.

But in Metroidvanias or Zelda-likes, getting new abilities and being able to explore further as a result is central to the experience and one of the strongest rewards it can provide. When done well, the new abilities are versatile and create new possibilities for combat and exploration, opening doors you didn’t even know were there.

If instead the new abilities have single niche uses to overcome specific, arbitrary, clearly delineated obstacles - they might as well be colored key cards for opening locked doors.