Matchmaking idea: Handicap
So, an idea came to me after watching a team be mercilessly beaten by 2 level 30s.
Team A was between levels 10-21 and Team B was generally under level 10, but two of its players were level 30. The two 30s dominated. So the idea I had was: When your teams are put together, "regress" higher level players to lower level equivalencies. If the enemy team is all lower than level 20, disable tier 3 runes. Set everyone's mastery points to the average team level.
You can make it more extreme and instead set everyone to the lowest person playing. If you're in a group with a level 1, everyone has one mastery point and no runes available. When going against higher level players, in theory they should be better than you because of experience, not experience plus runes and masteries.