This was a tough one. But I’m probably going to have to say JSawyer Ultimate for Fallout: New Vegas. A continuation of the game director’s own rebalance mod, it makes the game hard in a way that feels fresh and fair (unlike the baked-in Hardcore mode).

I like to pair it with my runner-up: JAM - Just Assorted Mods, for a more modern HUD and a sprint feature. Of course then you need the NPCs Sprint mod to rebalance combat and- you know what, all of the Viva New Vegas modlist while you’re at it ;)

I count Dark Souls as a Metroidvania in my head, honestly.

But I think the actual defining feature is unlocking new abilities to reach new locations.

DS1 has you unlocking new areas that are interconnected with ones previously explored, but you don’t really unlock new moves to get to a new place, it usually just happens after you beat a boss or buy an item.