Pretty simple, you have two teams. The arena would be a big volcano which you have to climb up. Players are given a 2 second head start. You can only build with either 2x2x1f bricks, 4x4x1f bricks, 1x1x1 bricks or 2x2x1 bricks.
Basically building takes points. You gain a point for your team by surviving for one second. If you die, you stay dead until the round is over. At the top of the build, there's a point where you have to touch in order to win the round.
You have to build to avoid dying via lava, and you have to build to escalate the volcano. You would also be armed with the pushbroom to push off enemy players.
Also, if possible, you could make it so one, bricks that aren't owned by your team cause damage to you when you stand on it, and two, you can destroy enemy bricks by clicking them.