I think you're right. Whilst, I don't think this would make a huge difference to the game - it's certainly more worthwhile than pumpkin heads.
There is no concept of a impact. Obviously you would associate lesser injuries with niggly fouls and more with head on collisions. I can't think programmatically, this would be too difficult to do. Given the speed/size of players are going into the tackle, the greater the chance of an injury/knock. You could also add factors such as tackles from behind and the injury proneness of a player.
Going retro, I personally always used to love the stretcher in the N64 versions of ISS. Thought that was a good touch.
Again, these small things are being overlooked - and I believe that they would add a little bit extra to a title. I also fondly remember the weather effects of N64. If you fully sprinted and tried to turn, sometimes you'd slip and fall over - maybe a bit OTT but I like that kind of thinking.
Whilst I'm all about the gameplay, I do think that presentation is important when it comes to football games. The game might play amazingly but if that 90th minute goal doesn't feel like a last minute winner, there's an issue there.