What do you guys think?
I think adding a mastery requirement is a horrible idea and ultimately pointless.
For starters, mastery points don't reflect player skill. You can have 3,000,000+ mastery points on your main and still not know what you're doing. In fact IIRC one of the highest mastery point Heimerdinger players in only Iron/Bronze.
Because mastery points don't adequately reflect player skill, all this does is add a very unnecessary time requirement. Let's just say for argument's sake that it takes 25 games to reach mastery 4 on a champion, which is already unlikely because that's assuming you get 800-1,000+ points per game and people are bound to lose a few times.
Anyways, that's 25 games for one champion. If someone plays 7 games of League per day, that's 4 days of League to be able to play that champion in ranked. Then they get banned, so now you have to dodge or get another champion to mastery 4. Another 4 days. Then the enemy team first picks that champion. Another 4 days.
It's just too much of a time investment for a free game, it turns new players off to the ranked ladder, discourages variety, and would just make the game a lot more tedious.