A Nami novel? She definitely has her post time skip outfit, so I assume it is some time after she was a student at Weatheria.
I would be intrigued by a novel about Vivi infiltrating Baroque Works, or literally anything between Nico Robin being between 10 and 30 sans the parts that were shown in the main Manga.
Koala joining the revolutionaries after the death of Fisher Tiger, and some missions with Sabo could be interesting, but I personally don't care too much about Koala.