A way to narrow the champions people use in Ranked.
Make it so you have to have the champion to Mastery ranked 3 before you can play them in Ranked.
It will solve the issue of people playing new champs in Ranked, the newly released champs being played in ranked issue, and it'll give some comfort because you know people will have at least some understanding at how to play the champ.
Since it takes a few wins and many loses to get to Ranked 3. You'll know that if they're ranked 3 there is a good percent chance that they've won with that Champion for a decent amount of matches.
Another way that it won't be much of an issue is now because the mastery system has been implemented for a while you'll have a decent pool of champions that are to Mastery 3. So anyone who is level 30 and playing the game should be able to use the characters they are good at in Ranked. And new players rising up through the ranks should be gaining Mastery levels for the champs before they even get to 30. So they'll have a pool of champs to be able to use. It also works because anyone who only has two champions with a high mastery rank means they probably only play those champs. And you'll probably only want them to play those champs as opposed to someone new in Ranked.