James Cameron wasn't actively involved in Fox' Terminator: The Sarah Connor Chronicles, but he must have been somewhat impressed with the direction the show took the characters he created because he's hired the series' mastermind Josh Friedman to work on the Avatar 2 screenplay. The Wrap broke the news, but unfortunately we don't really have any further details on what Friedman's involvement is or how the story for Avatar 2 is coming together.
Cameron's plans for Avatar sequels have been pretty fluid over the years, but here's what we do know about the film so far: Sam Worthington and Zoe Saldana will be returning. He wants to shoot it using a high frame rate, and wants Avatar 2 and 3 to film simultaneously. Both are supposed to be stand-alone stories that are related to the first film, but not necessarily intertwined (which is good, since it means part two won't end with a "to be continued"). And he's even teased that he has an idea for an Avatar 4, which would actually be a prequel to the first film and tell the story of what happened when humans first arrived on Pandora.
Cameron's own deadlines for the sequels seem to be a constantly moving target. Fox had hoped to have the first sequel in theaters by Christmas of 2015, but that may not be feasible if they take longer to get started. However, between the Friedman news (and we don't know if he's already done work on the script or if he was just hired) and the recent test footage that Cameron showed at a technology expo, it seems like someone's lit a fire under Cameron's feet. Hopefully that means we'll have some more concrete details soon.