This should be an easy one! It would be really useful if there was another tab on the player profiles (after Overview, Skills, Stats etc) that could be used to keep notes on players.

I'd use that kind of space to write notes on whether I thought the player was a fast trainer etc, or whether I wanted to sell them or keep them at the end of the season. It would allow a bit more planning as you go through each season.

The text would have to be private and would have to be deleted if the player was sold to another team.