Champion Diversity in competitive play
It would be REALLY cool if the same champions/compositions could not be used more than once in a single series. Meaning that all 10 champions on each side could only be used once per series. At the most, this would eliminate 42 champions from the pool of well over 200. It would make watching pro play a lot more interesting and would truly test who the best teams are, instead of testing which teams know how to play the best compositions.
Game 5 would be really cool. No more Gragas vs Rek'Sai 5 games in a row. People would have to actually expand their pools, play to their strengths, ban out certain players (for real, though). In addition, the "ADC" role would get some very much needed innovation, as by game 5 of a series you might actually be out of "ADC's" and have to cleverly delve into the "Marksman" role. I'm talking Teemo vs Kayle ADC bot lane, dogs and cats living together, total madness.