There are two ways to reduce variance in cup football results: on one side, split a 90 minutes game in, say, 3 x 30 minutes minigames (or sets like in tennis and volleyball); on the other side, make the cup a league or a superleague, even better with a regular season followed by playoffs (like in basketball, both NBA and Euroleague), much better with salary caps, etc. This would destroy football as the game we know, making it more similar to NFL but less prone to luck, flukes and single catastrophic events settling otherwise balanced battles.