Easy:
* Yawn: A common action, but why does someone yawn?
* Yearn: A feeling of longing, but for what?
* Yellow: A bright color, but what is hidden behind it?
Medium:
* Yolk: The center of an egg, but what secret does it hold?
* Yonder: A direction, but what lies beyond it?
* Youth: A stage of life, but what mysteries are hidden within it?
Hard:
* Yggdrasil: A mythical tree, but what secrets does it guard?
* Yantra: A symbolic diagram, but what does it reveal?
* Ygg: A Norse god, but what is his true purpose?
Bonus:
* Yggdrasil's roots: This could be a clue to something hidden or a place to search.
Remember, the best mystery words are those that spark curiosity and lead to interesting questions!