Require Mastery 3 or higher for Ranked play
Just to make sure someone at least knows the champion's abilities.
EDIT: It seems people are overestimating the amount of play required to get to M3. it's not much. And anyone who wants to play a champ in Ranked should have at least that much experience with the champion.
It doesn't reduce your effective pool. Your pool is not increased by a champion you own but can't play.
And maybe M2 instead. Smite requires M1 (out of 10) to play a champion in their ranked, which I think is still way too lenient but they have half as many characters as LoL. But the introduction of a Mastery system finally gives LoL a way to make sure someone has at least played a match with the champion they're choosing.