I think there needs to be better comeback mechanics, here's my solution
Problem: As the gold difference between teams increases, there's very little the defending team can do to come back into the game. Seeing drastic differences in kill counts, teams get discouraged and it's no longer fun playing a game that you know is going to end in a loss, you're just playing to the point of getting 3 other people to agree to concede or waiting for the enemy team to run it down mid and take your Nexus. Sometimes the defending team can come back from a devastating lead, but it doesn't happen often enough, and I'd say it happens less than 5% of the time in my games.
There's one thing that the defending team has access to that the winning team does not and that is super minions. While the winning team is controlling the map and taking objectives, the defending team gets to farm supers. Changes made to the rewards from killing supers can help the defending team get back into the game if the winning team doesn't press their advantages.
Proposal: Gold and XP gains from supers should be increased at least 3-fold if not more. XP gains shouldn't be split but have the same amount given to every champ in the vicinity (losing teams tend to be behind on XP). To give some additional help to the defending team, supers should also be immune to minion damage when an enemy champion is nearby (to allow a champion to secure the kill on it).