I have quite strong views on replays and like to take a lead from the way the yanks do it. In the NFL each head coach gets a red flag which he throws if he wishes to challenge a decision. If the original call is upheld, then he loses a time out which in the latter stages of a game can prove very costly. Translating this to football, I would allow managers to issue a challenge, which, if unsuccessful would remove the right to challenge for the rest of the game. The right to challenge would only be available in the case of red cards, penalties or goals. And would also be available in the case of decisions not given too. My caveate would be that once the ball has crossed the halfway line, or play has restarted after a stoppage, the right is lost. And, as in the NFL, it would have to be a clearly incorrect call to be reversed.
As for the red card for goal scoring opportunities, I'd do away with the penalty area and the red card for the offence. Instead, the ref decides whether the offence warrants a penalty based on the likelyhood of a goal being scored. How can it be right that a player on the deadball line running away from goal gets a penalty when brought down, yet a player clean through in front of goal just outside the box doesn't get one when brought down from behind (Vaughan v Cardiff).