Wrong Runes and Masteries
I am sure we have all been there, we get into game and to our surprise we notice that we forgot to switch the runes and masteries for the champion we are playing.
Although hopefully this isn't a common occurrence for you it is nonetheless a frustrating and game changing experience.
So I have a proposal for your riot. Allow players to preset their masteries, rune page, and summoner spells for certain champions. So if you played Leona support last game and this game you are playing Sivir as ADC your masteries, rune page, and summoner spells will be set to what you have preselected for the champion. So in case you forgot to change those settings during champion select you won't be handicapped with support characteristics as an ADC.
Of course certain champions can take different roles or modify their role depending on the situation they should still be able to change their masteries, rune page, and summoner spells in champion select so that they can fit their desired role. This proposal is just a safety measure to prevent mishaps when players forget to change their settings during champion select.
. You don't know the struggle until you're passive is doing more damage than your primary damage ability