This is what I personally think the plot should be for this movie. I highly suggest that if they do a third movie they must get and to reprise the roles of Sherlock and Watson, the two are amazing together and make the movie what it is.
Plot: The movie should take place a few years before the first movie, as it is a prequel. They should explain the origins of Sherlock, their own tales and personal life before they become partners. Younger, newer and more vulnerable to extreme cases, the two become entangled in the case of the Ripper, the famous serial killer in the United Kingdom who was active during the year of 1888. A young Sherlock and Watson must track down the killer, getting evidence, fighting goons and continually getting in to the worst of situations. Can the two track down the Ripper before he makes his last kill or will he outsmart the great duo?
I think it would be a great twist since the Ripper basically disappears. At the end, they believe to have him, but don't (something like that). I also suggest that (the man who played Bane in The Dark Knight Rises, play The Ripper). His role in that movie changed the trilogy and he can bring a whole new theme to the Sherlock Holmes movies. That's what I think.
What other actor choices or plot ideas do you have?