Two of best and greatest detectives ever know to the world - Sherlock Holmes and L Lawiet from Death Note. Both have two very different ways of deduction and only trust one person as real friends, John Watson and Watari. They both have a arch-nemeses in James Moriarty and Light Yagami and I think it would be great if there could work together in a crossover. It also seems they like sitting in the same way when thinking.