Gatiss and Moffat have often talked about making a lot of episodes of Sherlock over a long period of time, with the main actors eventually growing into the age at which their characters are normally portrayed (i.e. late-40s to early-50s). Assuming that everyone's schedules stay as jam-packed as they are currently, it's fair to assume that we may not get there, unless the show is revived many years down the line. In this case, a one-off episode looking into the future wouldn't be a bad thing. The series has pointed towards Holmes' retirement before, with a passing hint in His Last Vow to beehives on the Sussex Downs. And prosthetics are now sophisticated enough to turn even the most youthful of actors into wrinkly old codgers; Moffat did it with Matt Smith in The Time of the Doctor to pretty good effect. Finally, it would be a good way to present some of the weaker Holmes stories from the later parts of the canon, such as the utterly silly The Lion's Mane. Can we really see a young Sherlock Holmes taking on a murderous jellyfish?