Probably too late by now, but make the arena shaped like a funnel. The "sand" is actually small ball bearings imbedded in the floor. Balance checks needed to move around the arena, along with climb checks to move towards the outer edges of the arena.
An electrical current runs through the ball bearings. When you strike someone with a metal weapon, you add electric shock damage to your attack (+1d6). Flip side is if you strike someone wearing metal armor, you receive electric shock damage in the same amount. If you go for more than 2 rounds without discharging into someone else, you take 1d6 points of damage. Combatants with electric resistance take and can give no additional damage from the charge.
If everyone fails their checks and slides to the center, they all sit in the middle shocking each other to death.