I'd love to see either of these two implemented.
Video Referee - Like in Rugby, if a referee is at any point unsure of a crucial decision he can opt to go to the video referee to review his decision. The video judge then looks at the incident from all angles and looks for indisputable evidence to agree or overturn the decision.
Challenge Flags - Like in the NFL, it basically follows the same ideal of the Video Ref. Except the coach throws a flag when he believes the call on the field is wrong. They are limited to 2 flags per game but if both flags are used and the coach who thrown them won the challenge by having the decision overturned he earns an extra flag.
Something like this could solve so many issues. Penalty decisions, ball crossing the goaline, goals disallowed for being falsely called offside. It would be a great thing for the game I think. Not only would it combat these decisions but I've seen many times in rugby and football where these decisions can be turning points and reinvigorate a team which is an exicting prospect in my eyes.