Capcom has announced DLC for the Japanese version of Phoenix Wright: Ace Attorney - Dual Destinies. Although the US version has yet to release, it's likely that this content will also be released post-launch. Planned content will range from costumes, to Phoenix Wright's first case since the events of Apollo Justice: Ace Attorney.
Siliconera details the planned DLC releases. For example, there will be costumes to purchas: Phoenix Wright's original suit, Apollo's casual outfit, and Athena's schoolgirl outfit.
Most substantial is the "Special Chapter: Turnabout Return" DLC that will take place between Apollo Justice and Dual Destinies. In this case, Phoenix Wright is hired by a trainer dressed like a pirate to defend her orca, who's accused of murder. You could say that's one... killer whale.
None of this DLC has been announced for the US yet. However, the game is coming this fall to the 3DS eShop.