Here's another one that's been sitting in my brain for a while.

Do you ever look for the ant crawling up your leg, only to find that there's nothing there? Have you ever felt a bulk in your eye, but you swear there's nothing in it? Do you have thin, brittle hair that's constantly coming out? Fiery and intense growing pains? Slightly discolored skin? Undiscovered disease? Cancer?
Have you ever thought that maybe it's something else?
Maybe there is no ant. Maybe it was sweat dripping down your leg. Maybe the bulk in your eye is just too small to see. Maybe your hair is dry and brittle because you were born that way. Maybe you're in the middle a prolonged growth spurt. Maybe you're sick.
Maybe it's worse.
Maybe your veins are growing abnormally. Maybe your eye is getting bigger when it shouldn't. Maybe your hair is burning up. Maybe your bones are growing un-naturally. Maybe your skin is changing. Maybe it isn't disease.
Maybe it's mutation.

O.o This is one I really want to read or write, but if I write it, I won't finish any of my stories.
Okay, time for the pick up! If you're thinking,
"It sounds interesting, but I don't know how I would get to that point, so I'll just ignore this and move on." Then I have a treat for you! It's been in the mind attic so long, that I figured a way to that point! Yay!

A once successful scientist slumps into his/her chair. It'd been a long and devastating day. What else can you expect from a Kraang labor camp? I mean, what good could possibly come from endless weeks of researching and experimenting? But today was worse. Today he/she had actually helped the Kraang.
It was an accident, of course. He/she despised the Kraang, and did his/her best to destroy and ruin every batch of mutagen that got toted into his/her lab. This time, he/she took the mutagen completely apart, leaving only the chemicals not found on earth, figuring this would disable it. And it did—in a way.
It put the mutagen into a dormant state, inactive until paired with the missing chemicals, slowly mutating the host as each missing component was added to the mix in the bloodstream.
Why didn't I just throw out the foreign chemicals?
He/she thought desperately.
How much harm could it have done?
The scientist knew it could have been catastrophic, as he/she didn't know those chemicals and what they reacted to, but the didn't stop he/she from blaming his/her self for the disaster that would soon ensue…

As always, the summaries are just there to get the creative juices flowing, not because you have to use the whole idea, though you're welcome to.