Ranked Mastery Requirement
Solo queue is the closest this game comes to a professional scene outside of LCS, so why on earth would they allow a player to play a champion they have absolutely no mastery of in a role they've scarcely touched? That's terrible for their teammates and bad for the game in general. Since so many players do it, the overall quality of the ranked scene is worse for it.
Riot should make it a requirement that you have at least three champions at mastery seven before you enter ranked, and that the only champs available for you to play in ranked are the ones you've mastered to that level. That way, no one in ranked would be unskilled at their role and champion, and no games would be thrown by a player selecting a champion they have literally zero mastery points on for laughs.
It'd also reduce the number of smurfs in the game since they'd have to take the time to grind those champs they mained on another account up to seven yet again on their new account. Win/win, right?
It wouldn't even be a bad thing for a player who wants to change a role or champion. They could always spam a hundred games or so in normals as that champion and by the time they're ranked high enough for competitive play, they'll have become quite proficient at their new main.
Just a thought.