Yknow what we haven't seen much of lately? A serious attempt at the film noir detective. Downey would be very well suited to fill the shoes of Humphrey Bogart. Much more natural fit than Holmes.
I like the way you think.

Though, to be fair, the Holmes mysteries are much closer to American hard-boiled detective fiction than most of their English contemporaries.
In fact, I would be very interested to see a new Holmes film that played up exactly that angle, on the mean streets of late-Victorian London.