Supposing you had a time machine, what would you go back in time and change? I don't really mean big world history things like killing Hitler or stopping 9/11 - great ideas, sure, but probably a little beyond the scope of this exercise.
Think of something maybe more personal - like, the big fish you almost caught, or the time you jumped in the elevator and got it stuck. It's pretty much impossible to go through life without regrets, so take one of yours and give yourself a fictionalized redo. Try and imagine how things might actually have worked out; maybe it would have been better, maybe about the same, maybe even worse. Play it out as far as you like, but try to keep the word count under 700 words or so.