I really hope that Tom Hiddleston will play the 3rd Holmes brother. I imagine him as being as smart as the others, but he doesn't really care about being smart.. My headcannon is that he'll be really super nice and happy, like a puppy, and he just spends the whole season trying to get Mycroft and Sherlock to be friends. It would be such a good role for Tom, not to mention the fact that it could result in some sort of badass season finale where he could finally just break down and become a sort of Moriarty, feeling betrayed by his family, and wanting revenge.
I don't doubt that it will be amazing
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