There’s only been a few tweets about this, so I’m not comfortable calling this 100% confirmed, but looking over the accounts involved (primarily @earlbox and Gamestalk) there’s some history of accurate early Famitsu info, so I’ll put it out there.
According to the leaks, Famitsu reveals that Sherlock Holmes will appear in Dai Gyakuten Saiban (presumably as the rival character). This version’s Watson is an eight-year-old girl.
Both characters were silhouetted on the official site recently:
We’ll provide updates as they come in.
Update: enough other sources are picking this up that we’re calling it confirmed. From Gematsu:
Detective of the century Sherlock Holmes will appear in The Great Ace Attorney alongside eight-year-old M.D. and literary girl Iris Watson, this week’s Famitsu reveals.
Together with Holmes, protagonist Ryuunosuke Naruhodou will investigate incidents. However, that doesn’t seem to mean the pair will be together at all times.
Famitsu also introduces a new element called “Joint Reasoning.” As Naruhodou and Holmes work together to solve mysteries, players will point out where Holmes is taking his reasoning further than the truth in order to get to the bottom of what really happened.
As far as the game’s flow goes, it will be similar to previous entries in that it follows detective and courtroom parts. This time, however, it will have both 3D characters and 3D backgrounds. And the actual depth of the background will be utilized (meaning, the character can actually go into the background).
The Great Ace Attorney is due out for 3DS in Japan in 2015.
(Reminder that “The Great Ace Attorney” is not an official title.)
Hopefully scans will crop up in the next day or so!