Fog
When the time comes the whole arena is filled with a dense fog as if you are playing on below tiny render distance, making PvP harder and chest sight/chases harder to continue.
Katniss
When the time comes, random places, are bombarded with a 10x10 area of arrows. Making you 1.) lose your sprint. 2.) take damage.
Drop Party!
This is a positive disaster, when the time comes, and diamond/ingot and a stick are dropped right above the crafting table at spawn, making a rush for the item, and a bloodbath begin
Hope you like the ideas!