Hello Gang,
According to several sources around the internet, Rainmaker has officially moved on to working on Sly Cooper since they have finished the Ratchet & Clank movie. The people who have been saying this are fans who have sent private messages to Rainmaker on sites such as Facebook. They respond with similarly worded responses, always saying something along the lines of "Sly Cooper is next on our development slate." (Here's an example from Reddit.)
Anyway, it's good news, so stay tuned for more! (I have a feeling something will be at E3 this year... EDIT: I was clearly wrong.)
Teenbat,
bureaucrat for the Sly Cooper Wiki.