Carol Ann Duffy published a collection of poems from the perspective of the wife, sister or friend of famous, infamous and fictional men in history (Mrs Darwin/ Mrs Scrooge/ The Kray Sisters). I remember reading this collection when I was studying English in sixth form and being in awe at the idea of these poems. My personal favourite is 'Mrs Sisyphus', the wife of Sisyphus, the man whose punishment for cheating death twice was to spend his life pushing a huge boulder up a hill, for it to roll down just as it reached the top.
Check out the collection here: The World's Wife by Carol Ann Duffy (goodreads.com)
This idea inspired a lot of writing from me, including a piece written from the perspective of Alice in Wonderland, debating whether her trip to Wonderland was an adventure or just an overdose... Using real characters to inspire your writing isn't cheating, it's called being inspired and a great way to exercise your writing skill. So why not try writing from the perspective of a family member/ girlfriend/ boyfriend of a fictional character.
For more writing prompts: Blog | Sophie's Proof (sophiesproof.com)