It's annoying when my ST is the leading goal scorer in the league, but, because he doesn't have potential growth, he doesn't grow a single rating. Also, the potential locks when moving teams are on your Fifa 16 account. I signed Tony Vilhena 73 rated with a potential of 81 at 18 years old. He's been in excellent form for half a season and a regular starter, but hasn't grown in a single attribute. I know the loan glitch would most likely fix it, but I like to keep it as realistic as possible.
Injuries do harm potentials in my FIFA. My Martial never got past 83 after returning from a 5-month injury. This is someone who grew from 78 to 82 in one season. It was sad.
I like the idea of in game managers. I would love if they had certain personalities and tactics too. Imagine coming up against Mourinho with his defensive tactics to buy Fifa 16 coins. Maybe he leave Chelsea and you come up against him in a different league because he has taken up a job there. Also managers have players they like to work with and are more likely to sign them when they move teams!
The Growth system is fairly disappointing in FIFA. Players should grow based on how they perform. Someone like Depay isn't going to get better in real life by sitting on the bench all season and playing badly when he does come on, or by getting injured constantly. Players who do not play should not grow very much.
Of course players will often get better in real life over time. But players who perform well should grow. This can be scaled so for younger players (teens to early 20s), they can grow just with regular game time, but with players in the 22-27ish range, if they play well with consistent game time, then they will grow.
Agree with your point about the growth and performance/form correlation but I think that consistent game time for younger players (YA products/players with not a lot of experience) should also lead to some growth, albeit not at the same rate as if they were performing excellently.
There should be 'retirement leagues' like MLS, Saudi League, Qatari League and ISL where a legendary player should be able to get transferred when he crosses 33-34, instead of simply sitting in his old club and not getting a single game.