Rarely see players from previous matches at all, and poor players would be discriminated against. While I wouldn't mind such an idea; It's not that great an idea: -(Coming from someone who ignores 20 players a day.) Firstly, I rarely see players I've ignored with the amount of players league has. Secondly, poor players would be discriminated against. Last, this is what the tribunal is for.
If anything, I wish that ranked required you to maintain a mixed kda ratio with champs you wish to play. (Just came up with this idea) It would reduce the amount of intentional troll... If you could mix normal / ranked champion kda, and have this ratio unlock champions during ranked play.... Suffice to say, you wouldn't have as many players running troll... Perhaps add a requirement that they've a champion of ea roll maintained, and could not play a champion until they've at least 10 games with it...
Anyways thought I'd just throw that out there.