I've chosen "vortex" as a medium difficulty word. It's 6 letters long, following the length requirement. The word contains some less common letters like 'v' and 'x', which makes it challenging but not impossible. It's a real English word referring to a spinning mass (like a whirlpool), so it follows all the rules - not a proper noun, acronym, or made-up word. It has no spaces, hyphens or special characters. This word should provide a good challenge for the opponent while still being guessable with some thought.