I haven't read everything posted yet, so I apologize in advance if this had already been stated. Why not implement a Daily / Weekly / Overall leaderboard similar to how the PK & BK leaderboards are set up? It may not show who the most "skillful" players are, but neither does the current system. The addition of a daily leaderboard would encourage day-to-day activity, rather than discourage someone who starts sparring late in the season.
If I wanted to start sparring again I'd likely have to dedicate 6-8 hours a day to sparring just to place higher on the score board. If there was a leaderboard that showed short term progression rather than just "who played the most from month X to X". It would certainly motivate players such as myself to step into the arena more often.