Requiring a player to have a certain number games played (maybe even restricting it to a certain number of wins?) in the normal equivalent queue before that champion can be a valid selection for ranked play.
This would simultaneously stop people from playing new champions in Ranked Queues prior to actually obtaining any real gameplay experience on playing that champion, not just newly released champions but also existing champions who players for unknown reasons want to have their first game as that champion be a ranked game.
ImAPro wants to go into Ranked 5v5 Summoner's Rift. He has the 16 champion requirement met, however, he has only ever played Ashe on the Normal 5v5 Summoner's Rift. With the above requirement, the other 9 champions this player has, would be invalid selections and ImAPro would continue to be locked from the queue.
Enough of that, on to the next.
This same player has since played each of his 16 champions in Normal 5v5 Summoner's Rift and has played a few ranked games for that gametype. However, now he wants to play Ranked 3v3 Twisted Treeline.
He remains locked from it, why? Because he has does not have 16 valid selections for the queue, having never played a single Normal 3v3 Twisted Treeline game in his entire time on League.