1.)I had an idea regarding 'Experience'.
Say you were managing Liverpool, and last day of the season, had Tottenham away, whoever won clinched 4th, imagine Gerrard's experience in situations could somehow help influence his performance. Where as a younger player like Sterling would gain experience from it, but would be slightly perplexed by the sheer weight of the game, meaning you would have to find a balance of youth and experience like managers truly face when trying to find squad balance.
Also, say you won the title with Tottenham for the first time ever, people always mention it's harder to re-claim it then win it in the first place, so something like this would add to the head ache of being a manager. You have group of 21-25 year old first time League winners, but would they have the savvy to win it a second time? Surely needing experience and a level head would be attributes needed for the second season to hold the title? Something younger players wouldnt have but could learn with each success, giving the board and fans more understanding when you sign a 33 year old CM with 3 titles under his belt when you have 2-3 younger players who play well together in the positions, but that added Steele would be appreciated more.
2.)Also, another thing I'd love to incorporate would be players learning each others game, and bending to fit a style that suits them both, so Van Persie and Rooney at United gradually blending to becoming a better striker partnership. (The same with a GK and his Defenders, or two CB's, or 2 CM's or a Left back and Left winger and so on)
But then of course, certain players just wouldnt blend no matter what, such as being two similar or complete opposites (One's short and technical with 10-15 assists a season in him and the other is a bad finisher who acts better as a Target Man)