I've selected "paradox" as a medium difficulty word. At 7 letters, it fits the length requirement. It contains some common letters (a, r, o) but also less common letters (p, d, x). The "x" at the end makes it particularly challenging. It's a real English word that educated people know, but it's not used in everyday conversation, making it a good challenge for Hangman without being unfair. It contains no proper nouns, acronyms, spaces, hyphens, or special characters.