FIFA 15 isn't out until late September, but EA has already started developing the next game in the series. And the game after that. This has been revealed by EA boss Peter Moore in an interview with CVG.
"There are two phases to [development]. There's the core game itself. We have several hundred people working on that and it's a staggered development process, so there are already people working on FIFA 16."
"I think some people have the misconception that you finish the game and the entire team goes off for two weeks before starting on the next game, which isn't how it works."
Moore then goes on and reveals that FIFA 16 isn't the only football game they are working on, and there's FIFA 17 as well:
"You have parallel development that goes on where you have frontrunners that are thinking about what FIFA 16 is and in some instances FIFA 17 because there are development and engineering and tech decisions that need to be made."