Elaborating on this, it would be nice for the server or admins to be able to control who is spectating, and have a /forcespectating command. By doing this, only people who the server allows will be able to play, and everyone else has to watch. This could be good for 2vs2 games or something where you want to open it up and allow people to watch the action, but don't want to run any risk of them interfering.

Obviously, for this to work the server would need to enable plusonly.