I think a build contest would be epic. Just host a server, make it super flat, then have everyone build something according to a category. Then go around and pick your favorites. Then keep going like that 'til you found the best builder.
If you want to incorporate building and PvP, have a bunch of people build PvP arenas. The three who made the best arenas (or however you want to work the brackets) fight to the death in 3 games - one in each arena. Building the best arena is only the first part. To win, they have to win at PvP as well.