Among their jokes at the panel and talking around the possibility of a fifth season — "We can exclusively reveal that there won't be a fifth season... but there will be a sixth," said Gatiss — was one major reveal. Moderator Chris Hardwick asked the creators to give three words to describe the coming season. They instead gave three names: Thatcher, Smith, and Sherrinford.
Sherrinford was a name Sir Arthur Conan Doyle thought about giving to Sherlock before he created the character. After that, based on Doyle's notes, scholar William S. Baring-Gould proposed that it was the name of a hypothetical older brother of Mycroft and Sherlock, which would allow Mycroft to not be the oldest brother in the family, allowing him to plausibly enter public service. The eldest brother from a family of country squires would typical stay at home to manage family affairs. This theory was taken up by fans and it has subsequently found its way into non-canonical novels. The possibility of another Holmes brother was teased in the Season 3 finale by Mycroft, making the Sherrinford reveal quite, well, logical.