If a team has two challenges remaining - can they use them up with 5 mins to go , appealing for a thrown in for example, to waste time?
The more I think about this, you have a point. That towards the end of a match the losing team could challenge random things like throw-ins just to prolong the match. There should be limits to what can be challenged to avoid this problem.
On the other hand, there should also be a wider scope of decisions that should be allowed to be challenged that VAR does not allow to be challenged, particularly on decisions that lead to goals being scored. And unlike with VAR, with a challenge, you should be allowed challenge a corner kick / goal kick decision or a foul / non-foul decision that led to the goal even after a goal was scored.
Managers should be allowed to challenge pretty much anything (throw-ins, corner/goal kick decisions) as long as it directly leads to a goal. So if a decision, even a small decision like a throw-in, does not immediately lead to a goal, then they shouldn't be allowed to challenge it. This would avoid petty pointless late game challenges by the side that is losing.
The distinction for how far back you should be able to challenge something like an out of play decision (throw-in, corner/goal kick) after a goal is scored should be something like, if the ball (i.e. from an incorrectly awarded corner or throw-in) was cleared out of the defensive half of the pitch, or if the defense gains possession of the incorrectly awarded throw-in or corner and completes a number of passes.
This would have to be thought out by the powers that be, and I have no confidence that they'll craft it in a way that avoids problems. But if you're asking me how I would design the challenge system in football to replace VAR, I would make sure to set boundaries of what is allowed to be challenged to avoid undesirable or petty challenges whilst increasing the breadth of what could be challenged within certain constraints as compared to VAR.
In addition to challenges being allowed if an incorrect decision like a corner or a throw-in leads directly to a goal. I would allow a manager to challenge a missed non-carded foul in the opponents half that would have resulted in a goal scoring opportunity free kick.
It's a slippery slope on what could be allowed and what couldn't in such a system, and we would need to think long and hard about how best to design such a system. I would also allow a manager to challenge an incorrectly given goal kick if he so desires, but he would have to be quick about it, like if he sees an appeal from a player who signals to the manager that it should have been a corner. Maybe in a stoppage time situation that could give the team down by a goal a corner that was incorrectly ruled a goal kick.
In thinking about this, maybe do something like this. Allow each manager exactly 1 challenge per half, and if the manager wins the challenge then he is allowed 1 additional challenge that half, that would not carry over to the 2nd half or extra time. If you use up the one challenge you have, challenging anything you want, then you would have no more challenges that half.
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