Sherlock Holmes & Watson Vs Dexter Morgan
Robbert Downey Jr Holmes and Jude Law Watson are droped in Dexterverse
Original Sherlock Holmes and Watson are dropped in a paralel Dexterverse
(I do this split up in case Dexter decides to go on the offensive, seen as these two would very difrent results)
They do so during paralel Season 1, 2, 3 and 4.
They get modern day tech assitance, but, no initiative from the law enforcement agencies. What this means is that they'll answer question and run lab results, but Dexter is there to screw things up, and anytime they ask for back up they'll be hopelessly late.
This also means there is the ice truck killer, lundy, miguel prado and his family to worry about, in every way he does in these situations.
Absolutely everyone is in character.
After that, bonus round. The looser dies and professor moriarty cames to miami, with whatever extra knowledge he'dd reasonably have from being of this day and age, and he's on the side of the looser.