There are so many fascinating fragments of story tucked away in the LOTR appendices! Pick one or more of them and elaborate. It would be especially interesting to do something with less well-known stories or characters.

Some possible ideas: the life of Finduilas of Dol Amroth, Dís and other Dwarf-women, Valacar and Vidumavi, the death of Ondoher and his sons, Mardil Voronwë trying to figure out what to do after Eärnur's disappearance, the fate of Baldor son of Brego.