I think their whole shooting system needs balancing properly. The finesse shots in '14 were initially too easy to score, so to counter it they made it impossible. Just little things that need to be tweaked. Hopefully with the new graphics, animations, atmospheres and so on, '15 should easily be the best football game of all time.
Content wise, I would like to see them expand on the internationals a bit more, '14 was better with it being implemented into career mode, few more international teams wouldn't go amiss. And build on the career mode implementation of the internationals, make bigger deals of the big tournaments and "will they, won't they" scenarios regarding qualification and picking squads.
Also in career mode, a few more managerial options like selecting friendlies instead of just being given 3 generic friendlies, I want to choose who my team face pre-season and perhaps implementing some pre-season tour like things like the bigger clubs do in real life.