Champion Mastery
Perhaps allow it to slowly decay over time, to force players to keep up with the mastery on a specific champion, rather than just getting it to lvl 5 then ignoring the champion until the cap is raised or your tired of bringing up another champion to lvl 5. Also this will cause players who haven't gotten on for 3 months or more to have maybe 2-3 lvls taken off each champion with a mastery cp in it.