You wish to peer into the twisted depths of my imagination? Very well, then.
*cracks knuckles*
I know you said we were limited to one change, but I couldn't help but think of a few possibilities. Just consider them to be a set of mutually exclusive options.
-Whenever someone is alone, there is a random chance that a clown in tattered clothes, faded makeup, and yellowed teeth will appear behind him/her. The clown will then follow the person, its breathing gradually growing louder, until the person turns around and sees it. Then the clown utters something with a raspy voice in a language that the person doesn't understand before transforming into a swarm of large moths that scatter in all directions. Afterwards, the person finds themselves physically unable to communicate the experience to anyone else.
-Shortly after a person falls asleep, a group of tentacles grow out of the ground and gently wrap around his/her body. It doesn't matter where or on what surface he/she sleeps, the tentacles will appear. They do not harm the person in any way; they don't strangle people, they don't block breathing, and their grasp isn't strong enough to restrict even a baby's movements. They simply writhe across a person's body until he/she wakes up and breaks away from their embrace, at which point they emit an unearthly shriek and wither into dust.
-Nobody is able to detect anyone else's presence except for that "feeling of being watched". It's not that they're invisible or anything, but nearly all sensory information regarding other human beings simply doesn't register to anyone. Each and every person thinks that they're completely alone, but can't help but feel watched.
-The British are the chefs, the Swiss are the lovers, the French are the mechanics, the Italians make everything run on time, and the Germans are the police.
-At random intervals, but never less than one or more than six hours apart, people hear a voice whisper something in their ear. The messages are always short, cryptic phrases like "We are always watching", "You cannot escape us", or "Soon, very soon", and sometimes it's just unintelligible gibberish. When the voice is heard is unique to each person, i.e. not everybody is hearing the voice at the same time.
-Everyone is, at all times, acutely aware of every single microorganism living on their skin.
-There are numerous statues everywhere, there is no place anyone can go where there isn't at least one statue within visual range. Whenever someone looks away from a statue, it is in a different pose when they look back, a la the Weeping Angels from Doctor Who, except they don't attack anyone. The more times a person looks at a particular statue, the more disturbing its poses become; such as inflicting bodily harm upon itself, or gnawing the flesh off of a human arm. Eventually the statue will reach the point where, when the person looks back at it, it's gone. When they turn around, they find the statue there, staring at them. The statue resets the cycle at this point.
I may add more later if I come up with any other ideas.
