Just like someone told to select an arbitrary password might select any sequence of characters. They might theoretically select anything, but most of them will choose something like 'password'.
So with your users. Star Trek fans are going to choose "makeitso". And a database of famous quotes will catch them.
What I'm really getting at, though, is that I think playing cat and mouse with professional hackers is a losing game. You shouldn't spend a few seconds trying to come up with something that they won't think of when it's their entire vocation. You're just not that creative, and too many people think alike.
Just roll dice. That way your choice is provably random.
Just like someone told to select an arbitrary password might select any sequence of characters. They might theoretically select anything, but most of them will choose something like 'password'.
So with your users. Star Trek fans are going to choose "makeitso". And a database of famous quotes will catch them.