It's basically being developed by the Investigations team. Takumi will not have a major role in the development.
It will come out on 3DS.
Already confirmed for a Western release, however there are currently no release dates for any country.
Phoenix will be the main character, but Eshiro (the producer) has a made a point of the fact that Apollo Justice has not been retconned.
Some minor plot points released as publicity:
There will be a new side-kick; Maya is apparently in training in Kurain.
This game will take place one year after Apollo Justice.
The first case will be about a bombing in the Courthouse.
Payne's younger brother is the first prosecutor.
The theme of the story is "dark days for the law"; this is following from Phoenix's comment in case four of Apollo Justice. The chain on Phoenix's jacket is from the locket that has Trucy's picture in it.
There will be more information released during TGS.