I like the idea of live substitutions, it rewards the managers that can be bothered to watch their teams live. So often I have watched my matches & seen an underperforming player make error after error, and sat there wishing I could replace him - only to see the guy doing great on the other wing replaced.
The ability to change your formation would be good too. Sometimes, when a formation is just not working for you, it would be good to be able to change mid-game.
Other reasons you may want to change strategy mid-game, is if your gun midfielder is injured or sent off. The ability to slot the winger into the DM or central position & put the weaker substitute out on the wing would be extremely useful.