More masteries
Simple. Maybe lvl 10 Make after 5 actually hard. Getting S's can be difficult but I feel after you get those it should simply be back to points. A lot of points. Enough that it actually means you have mastery over someone. Maybe lvl ten could be something insane like 10 S+ tokens.
This would add more motivation to play one champ and stop people from quickly getting mastery without having mastery.