I fervently believe that soccer would be a much more entertaining game if the substitution rules were significantly loosened. When I played in high school, substitution was allowed on any dead ball situation, and we effectively used something like a hockey line shift, with midfielders shuttling in and out every five minutes. The result was that you could play at a much higher tempo than if you had to conserve energy for the entire match. It was far more enjoyable both as a player and a spectator.