Ok what about the following ideas (sorry if these have been discussed before - just say! - plus i have no idea how hard these things could be to implement if there was a desire)
Team chemistry - this could be a game performance modifier for all players (a bit like game shape is to individual players). one way of looking at this could be that established players in a team play better on average than similar new players to a team. the new player gradually settles and then perform like any other established player after so many weeks.
Player traits or personality types. Certain types could complement or not work well with other types.
maybe fans like to see certain types of players with traits/personalities so there could be indirect / subtle effects on attendances or merchandise which would keep the game more intriguing and a bit of a mystery!!
these ideas im sure could be developed some more on the forums for fun, even if it would just be impossible/low priority to implement!