I've had the best luck with cycling through all, but emphasizing physical (defense) and technique (offense)…at least with the default players.
What I REALLY miss from last year are the weekly reports of player stat improvements.
Also, because I've seen it mentioned, I have seen improvement in a few of the better players - Shimizu, Ettori, Palmieri (I think everyone has the same default youth team players), but most players aren't improving.
The original ML players don't have much room for improvement. A few of them do have the potential to get better but for the most part they stay the same. At least that's what I've seen so far, though to be fair I have to say I had replaced my eleven by the beginning-middle of the second season.