Sure, Sherlock would be brilliant in the TARDIS. At first. But, isnt he too much like the Doctor? Theyd pretty much finish each-others sentences. The audience wouldnt have a clue what was going on because there wouldnt be a not-so-clever assistant to ask all the important questions that guide us through the episodes. Thats if there was actually an episode to watch: the pair would solve all of the problems before the titles had even begun. Sherlock might be an interesting addition for a one-off episode. The power struggle between the pair would make for good watching. But would it really work full time? Probably not. Hes perfect, yes. But as a replacement for the Doctor not an assistant.