In the previous games, it's been impossible to tell whether a player has picked up a light or a bad injury during a game. What I would like to see in FM13 is a physio report after the injury, saying roughly what kind of injury he believes it to be.
The injury system should also get a lot more dynamic, with recovery times being affected by whether or not the player needs surgery, and that injuries might take longer to heal than what you first thought.
Injury-prone players should be more prone to some injuries than others. Say if a player has broken the same leg twice, then there is a huge risk that if he breaks the same leg he will have to retire.
These are all realistic features I would like to see.