since level 7 is pretty easy to get, and it dosen't matter if you get those s in blindpick or rank, I don't think "beeing lvl 7" at x champion means to much, other then you played few games on it, and somehow managed to get S on it.
So, I was thinking that a special level 8 mastery, that would be harder to obtain, and mean anything, like making 5 s in rank game and getting 1 pentakill, that would unlock the level 8 mastery. At least, then when you would see someone with lvl 8 mastery, you know he got those in rank game, and he managed to penta with his campion. ( Not sure if support mains, that are not intended to make kills, will agree, but... I don't know, maybe a "special" rule can be applied to support champion)
edit. " for support, there could be made something that is similar to a pentakill, in the ammount of assists in a single game. I gave a random number, 35 assists in a game, if that is as hard as a pentakill I don't really know, but something like that should be fine. "
> So my intetion behind the level 8 mastery is to show some skill, not be something that basically is just "grinding", nothing more then that.