Trying my hand at a crime novel

With "Not Too Late for Paradise"completed and available on Amazon by the end of this week, and "Narragansett" bogged down in final edits and changes, I decided to start my first effort at a crime saga. I got a couple of great characters, of course, one is based on me – a bedridden victim of ALS who is able to control his world with advanced technology. Kind of like Denzel Washington in "The Bone Collector" and his partner is similar to Angelina Jolie. They solve crimes out of a sense of justice and altruism. I've written the first five chapters – the characters are good and the technology is fun. But now I'm struggling with the back story and the main plot. I want the plot and story and characters to be engaging, but I also don't want the story to be too dark. I'm thinking the main bad guys might be a very successful thief – perhaps even a woman. Crime novels are much more difficult to write than I thought! Brings out great respect for other terrific crime and suspense writers that I have read.