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.