What is a good storyline for a crime fiction story with detectives?

For a writer, its very important to learn how to come up with ideas.

Writing Prompts are good start: https://sites.google.com/site/allusionsanddreams/h...

Creating your own random plot generator is another good way to come up with your own ideas. https://sites.google.com/site/allusionsanddreams/h... (different page from above)

Try and have as many characters as possible, and mislead the reader many times! The large number of characters will keep the reader guessing as to who the criminal is. Classic plots are bad guy steals something and sets up trap for good guy but is caught, or classic murder mystery. Read some classic mystery books like Sherlock Holmes or Nancy Drew for inspiration.

That's you basically asking for a story.