I loved Simm as the master, he was definately my favourite. But maybe a darker version would work well. The thing is, the master has always been the anti current doctor. For example, the first master was an evil version of Jon Pertwee, the green master/s were just dead corpses, the second actual master was just a copy of the first, but an improved coppy, the third master was basicly the anti Paul McGann, and Simm was the evil David Tennant, and Tennant was a loud and vibrant doctor.
Seeing Smith is not so loud, I thin it would work for Simm to calm down, however I did enjoy the ,adness Simm brought to the role. One thing's for sure, I don't think getting rid of Simm as soon as Tennant leaves is a bad idea.