I think u should use ur champion achievement system to prevent some player to do this.
To reach lv 5, only take about 30 games. If people want to use this champion to rank, they should practiced at least 20 games.
In other word, at least reach lv 3 or 4.
This is only an issue in lower elos. By mid-high elos (heck, it's a non-issue in silver II+ in my experience), people are competent enough to quickly grasp the workings of a champion. (Seen too many people pick off-champions and still do fine.)
Also as Deep mentioned, you'd have to do it for 16 champions MINIMUM, and with your ~30 game guess, that's minimum 480 games JUST to do ranked (excluding the grind to level 30, getting good runes) or about 240 hours of gameplay.
This only serves to provide WAAAY more frustration than someone first-timing a champion could ever provide (Again, it's only an issue in low elos. Nobody in higher elos wants this.)
If they required you to have a certain mastery level, they'd also have to make you do it on a minimum of 16 champions, and that's bullshit. I have one champion at level 5 (Nami), and until recently didn't even have anything else at 4 (but now Blitz and Soraka). Other than those, I have only 1 at level 3 (Gangplank).
So basically, even with your minimum standard of Mastery 3, I am only 1/3rd of the way through being able to even enter ranked play, yet I've climbed more than an entire tier from last season across almost 150 games. Why would Riot kick me out?
Your system targets a certain type of person, yet punishes everyone by default for no gain. Mastery level doesn't even equate to proficiency.
