Taking into account how prominent Ralph Fiennes and Hugh Laurie are as actors, that makes it very likely that one of them (I’m guessing Hugh Laurie) will play Professor James Moriarty. Although Moriarty only appeared directly in “The Final Problem,” he’s commonly known as Holmes’ arch-nemesis, and has most recently been played in live action by Jared Harris (Sherlock Holmes: A Game of Shadows), Andrew Scott (Sherlock) and Natalie Dormer (Elementary, as a hybrid of Moriarty and Irene Adler).
Although Arthur Conan Doyle’s Sherlock Holmes canon stretches across 4 novels and 56 short stories, there weren’t that many recurring/notable supporting characters.
Even though Holmes & Watson is a comedy, it would be weird if Ferrell’s Sherlock wasn’t matching wits with his intellectual (albeit malevolent) equal.