Over the years we have noticed trends in what is being published in the mystery genre. Some years we seem to have an abundance of books around the theme of kidnappings, other years it is psychological thrillers or serial killers. Police procedurals and historical mysteries continue to be huge sellers.
Short stories are challenging because you always have to think up a new beginning, a new character, a new plot. Novels are slightly easier as you have something that you settle in with -- something larger to explore and study.