I have an idea about the substitution subject i think its easy and possible to do, due to the factors that are already simulated in fifa 2005.
My idea is to change the way, not only visual but also tactical , of doing a substitution in the tactical aspect and really important is, in the substitutions menu add the option to put the players that you think need to play go run and do exercises to raise his physic form, that is already simulated in fifa 2005 , to prevent injuries and improve them before they start to play.
on the visual aspect they need to add the substitution animation showing the player that is going to play an the one that had been substituted. Also a substitution occurs when a player is tired or injury , so they need to add animations of the medical staff going in to the field to check de injuried player and after the player is checked by the doctor its necessary to know the player situation that also is simulated in fifa 2005 after you played the game in career mode the medical staff tells you about the injury the player have, but i think they need to advise you directly in game to take your choices , let the player play if is really necessary or substitute him Then they also need to add a animation of player tired asking to be substituted, this is also possible because its also simulated already in fifa 2005 they only need to create the animations.
This is my humble opinion about this subject and as mentioned before i think it is really possible.