Nationality and age on players?
Nationality on players:
Like putting nationalities on every player, so for instance, if you have a team with 5 danish players, the team chemistry would be 100% and the team would perform better, than if you had 3 danish players, 1 korean and 1 american. Then it would only be 60% maybe. Or 1 danish, 1 german, 1 french, 2 americans and it would be 50%. (I think you get the idea now)
Age on players:
So it's 'scientificly' proven that a gamers reflexes starts to decrease when they hit the age of 25, so my idea is, if a player is 16, he would be way more efficient during trainingsessions until hitting maybe the age of 20, going more standard until hitting 25 and from there, the training will be less effective and technique and such will start to slowly decrease and the training efficiency will be less great than before.
Just some a few ideas that came to mind, hope you'll like them and maybe consider adding them to the game (which I by the way think is awesome! good job)
Oh, and I still think the idea of being able to put a nationality on your team (or manager) would be great just to personalize it a little bit :)