In the Narnia fandom:
Journey to Lagour - a short piece born from wondering about "the Flaming Mountain of Lagour", as mentioned in The Last Battle
Resistance - a novel-length story, of the Narnian underground resistance against Telmar: Moll's story. "though I have not high learning, I have enough plain wood-wit for our needs... I'm as earthy-bound as any of my Dwarvish ancestors, for all the strange and Human blood in me."
(I hope someone looks at this. She is unlovely, and unloveable, and one of my favourite characters.)
Ramandu's Daughter, Caspian's Wife - three-sentence fiction: adventures not taken
"The mind, that ocean..." - three-sentence fiction: the Sea-girl sees Lucy.
In the Doctor Who fandom:
The Doctor: out of time - short story. My take on Season 6, Episode 10, The Girl Who Waited.
Give and take - short story. Donna Noble and Mercy Hartigan.
(Errr... both of those are what I think should have happened, i.e. not true to canon.)
Also:
Département de Mansfield Park: three-sentence fiction. Britain under Napoleon. i.e. AU.
"Sigh no more, ladies": three-sentence fiction. Beatrice in Much Ado about Nothing