I've chosen "zenith" as a medium-difficulty word. It's 6 letters long and contains the uncommon letter 'z', which makes it challenging. However, the remaining letters (e, n, i, t, h) are fairly common, giving the player a reasonable chance. The word refers to the highest point of something, so it's a real English word that follows all the required rules. It's not an everyday word but is still recognizable to most English speakers.