The Challenge mechanic is what makes ToI work so smoothly, but it's a tool I think could be used in a lot of RPGs. Challenges are the way base running and fielding works. A baserunner challenges the catcherʹs arm when trying to steal a base. A fielder challenges a hitter by trying for a spectacular catch on a hit. Players only have two challenges per game, so they need to pick and choose their challenges. Challenge a single in the third inning, and itʹs a waste. Challenge a homer in the ninth and you may not have enough to stop it. Otherwise, the result of the contest between a hitter and pitcher stands..
Say it's the late innings, one out, and there's a runner on third - a double play will end the inning, but if you can stop it, a run will score. A baserunner could Challenge that 6-4-3 double play, by taking out the second baseman and breaking up the relay, or by beating that relay out at first. If the runner from first is trying to take out the second baseman, he'd roll his Baserunning Advance check, while the second baseman rolls his Glove Ball-handling check. If the result is a tie or the second baseman wins, he gets the throw off before the runner slides into him. If the runner wins, the throw to first will be late or draw the fielder off the bag. The batter could also Challenge the relay by beating an Arm Accuracy Check from the second baseman with a Baserunning Advance check of his own.
Say there's a no-hitter going in the eighth inning, but the pitcher lets up a clean single to center. The center fielder can Challenge that single by beating the batter's batting check - which he just made vs the pitcher - with a Glove Ball-handling check. If he succeeds, he makes a circus catch and saves the no-no.
Challenges are an extremely flexible way to manage any situation where a result is establihed but a character will do anything to stop it. By limiting the number of Challenges allowed, you can prevent overuse. The characters can only Challenge things they really care deeply about.