Language from an advertisement for a recruitment drive strongly suggests that developer Avalanche is working on the next Just Cause game, the third in the series.
Avalanche held a recruitment event at the Crowne Plaza hotel in Baltimore to bulk up for multiple projects for current and next-gen consoles, which they hope to release in 2013 and 2014, respectively.
According to Eurogamer, Avalanche described one of the projects as a "ground-breaking" open-world action-adventure game... Whether you thought it was ground-breaking or not, that sounds an awful lot like Just Cause. It's unclear, however, whether that would be the 2013 or 2014-dated project.
Interestingly, Avalanche and Square-Enix recently bought a series of URLs related to Just Cause 4, despite having not even announced a third game. Could it be possible that the studio is making both games simultaneously?
Before jumping to any conclusions, remember that the studio is also supposedly working on a similar type of game based on the Mad Max film series.