The news comes from a job listing on Treasure's website, seeking a programmer for a "new project '3D action shooting game.'" In addition, a cryptic image on the front page, advertising the job listing, looks like it might be some kind of hint.
Treasure has made four games in the past that would fit in this genre: the PS2's Silpheed: The Lost Planet, the licensed Dragon Drive: D-Masters Shot on GameCube, and the two Sin & Punishment games. It is, however, a company that usually expresses more interest in original games than sequels, so past games are not necessarily any indication of future plans. Perhaps it's the company's long-awaited Xbox 360 shooter -- although that one was originally meant to be 2D.