Here are some options, depending on how broadly you define "positive":
General Positive:
* X-cellent: A playful way to say "excellent"
* X-tra: Can mean extra or additional, which can be positive in many contexts.
More Specific:
* X-ray vision: A superhero power, often associated with clarity and insight.
* X marks the spot: Can be seen as positive if it refers to finding something valuable or exciting.
Words with a positive connotation but not strictly positive:
* Xenial: Friendly and hospitable to strangers, which can be seen as a positive trait.
* Xerophil: A plant that thrives in dry conditions. This could be considered positive for the plant, though not necessarily for humans.
Let me know if you'd like me to explore any of these further or if you're looking for something else entirely!