Basically, all Science Fiction begins with asking the question "What if...?"
A variation, often called Alternative Reality or Speculative Fiction focuses, typically, on a key even in history and explores what might have happened if things had gone differently.What if there was life on other planets?
What if World War Three began tomorrow?
What if Martians helped build the Pyramids?
What if there were no distinctions between the sexes?
What if the Axis had been victorious in WW2?
What if Lincoln hadn't been assassinated?
What if Asians instead of Europeans had colonized the Americas?
Now we twist this, a little, in the MSF way. Take a prominent historical or literary figure and ask "What if ______ had been born a boy (or girl) instead of a girl (or boy)?"
What if it was Adelle Hiter, instead of Adolf? Would she have succeeded in conquering/uniting Europe, or would she have forgone war entirely?
What if Lady Diana had been Lord Dian, instead?
What if the first president of the US was Georgia Washington instead?
What if it was Eve and Adam instead? Would the matriarchy that developed be much different than the patriarchy of today?
What if Mary of Tudor (Bloody Mary) had been a George, Charles, or Edward instead (is there a male version of the name "Mary?"). Would his attempts at changing England's official religion have suceeded and/or not been immediately overturned by his successor?
What if it had been The Lion, the Wizard, and the Wardrobe?
What if Frodo had been an advernturous girl hobbit? Would the 'relationship that never happened' (Frodo/Sam) have happened? Would she have been the Ringbearer, or would others have thought she couldn't've handled it?
What if Jesus was a girl? How would religion be different today?
This is more of a thought exercise than a writing exercise. Rather than a piece of prose, dwell on what might have turned out differently, either in the individual's lifetime, or as a consequence of them being born of the other sex. Then, write a few of your thoughts down, perhaps in an essay-ish or outline form.