Something along the lines of Battlegrounds, except without all the killing and debuffing and buffing.
Sports that would work in RO:
- Poring Soccer
- Poring Tennis
- Poring Volleyball
What would happen is that you'd go to a sports NPC to enter the Sports Arena (like the Battlegrounds atrium) and go register for one of the sports available. Once both teams are assembled, the game begins! There could also be spectator registration for players who want to watch a game. A queuing system similar to that of Battlegrounds could be utilized to notify players of games.
Then there could be a tournament mode that activates once a year or so, and the winning team gets a trophy item along with a special headgear or garment to commemorate their victory.