Final Fantasy 14 director Naoki Yoshida is working on a "next generation" project at Square Enix's new 3rd Development Headquarters. Large-scale development has kicked off, and the team's now looking for new designers and programmers.
"Based on the former 5th Business Division, the newly launched 3rd Development Headquarters has launched a new core project following Final Fantasy 15, and has begun to challenge the next generation," reads the recruitment page. "This project has already finished its initial development, has finished developing its development environment, and is now shifting to a full-scale large-scale development phase."
Final Fantasy 15 launched on consoles in 2016 and, with its final piece of DLC launching just recently, it's now complete. Well, as complete as it's going to get. Unfortunately, other the DLC episodes were cancelled. Final Fantasy 16 seems like a pretty conservative guess, then, especially as it's a "core project".
Hopefully it won't be another decade-long project, though. Noctis' adventure started life as a Final Fantasy 13 spin-off until, six years later, it was turned into the next main game in the series. It took another four years to finish, and then three more before all the DLC was released.