Settle down children, this could get mildly elaborate. *clears throat*
A 3rd person sandbox game (a la GTA) set in OTT Dickensian London, where you play some kind of flamboyant, moustachioed detective - tracking down a string of increasingly mysterious occurences, culminating in the revelation that you are in fact a sort of "Mr.Hyde" figure, causing a dual storyline following the violent cowboy/vigilante justice of "Mr.Hyde" coupled with the By-the-book antics of the detective struggling to keep his alter ego under check. And get this - the big boss fight could be against YOURSELF!!!
Yeah, this wasn't one sentence long (I got a bit carried away) but I think my verbal diarrhoea is warranted by the awesomeness of this idea. Also, I will willingly accept any similarities drawn between this story, and that of the FX tv series "DEXTER"