I can't remember, exactly, where the idea came from. A joke, I suspect. Conversations about Shikamaru, Lee and Pakkun travelling in a van and solving mysteries.
Shikamaru is undoubtedly my favourite Naruto character. He's cool and clever, and not all about winning at all. In a series filled with people being loud and demanding attention, there's something refreshing about a competent character that wants to actively avoid the limelight. His lazy attitude towards life transfers nicely into the tired thing that noir detectives tends towards. That's before you consider his razor sharp intelligence and it's relationship with the word "detective". It was almost too easy a fit.
Lee, as the series goes, is about as natural a Watson as you could wish for. He's got the optimism and "youth" to bounce off the more hardbitten and cynical Shika, and is just more prone to just asking questions about everything. He's the brawn to Shika's brain - an almost custom made foil.
Welcome Shikamaru P.I. I promise you this: at the very least, it's gonna be one hell of a weird ride.