Benedict Cumberbatch or Harrison Ford -- or maybe both!
I'd love to see Harrison Ford in a villainous role, as I think he could really do wonders with it. I'd like to maybe see him play a convict on death row, or a murderous psychopath.
As for Benedict - I'd like to see him playing a bookish character in some sort of comedy. I know he has comedy chops and I'd quite like to see him use them