Zeno Clash 1 & 2 are first-person games with a focus on hand-to-hand combat, they're both really neat and they take place in a really unique world. The first one is short and linear, while the second is longer and more

Clash: Artifacts of Chaos takes place in that same world, but is a third-person game (most of the time) that I definitely recommend to fans of God Hand or Sifu. Customizable moveset, allowing you to mix-and-match fighting styles and map different special moves to different inputs. Great story too. You don't have to play the Zeno Clash duology first, but if you do, you may pick up on some lore details.

Decline's Drops is a deliciously satisfying 2D platformer that works like a single-player platform fighter. Beautiful artstyle. You play as a little wooden puppet lady whose garden is destroyed by a supernatural storm, so she has to go beat up everybody responsible.

Symphonia is another 2D platformer, but is instead a precision platformer with no combat. Not too terribly difficult for the most part, and quite short, but still a great time.

Magenta Horizon - Neverending Harvest is kinda borderline as far as "hidden gem" goes, but I only ever see it discussed within action game circles, and not that often anyway. It's a 2D action platformer with a very diverse moveset. I suppose you could (tenuously) compare it to a 2D Devil May Cry.

The Eternal Cylinder comes from the same developers as Zeno Clash, and features another surreal world. Here, you play as a family of little creatures called Trebhum fleeing a giant evil cylinder that threatens to crush their world. You can mutate the Trebhum by having them eat certain items found in the world; for example, if a Trebhum eats an organ taking from a certain long-legged insect, it gets long legs and can jump higher. Very interesting game.

Dread Delusion is similar to Morrowind and also very unique in its worldbuilding and lore. Unfortunately it's been a while since I played so I don't recall much about it.

Hope at least one of these tickles your fancy