I like the idea of Robot Wars, how your surroundings are deadly and could be used to your advantage. This could be used for PVP or Survival Arenas.
Examples:
Giant spider: Anyone in it's web will be slowed and every now and then she will shackle a nearby enemy and drag them closer.
Lava Pit: Deals major damage to anyone standing in it.
Spike Field: Random spikes erupt from the ground and damage enemies.
Rising Acid(from Samus' Stage in SSB): Acid rises from the ground burning anyone who doesn't escape to higher ground.
Random Weather: Windy (Global Slow), Infernal Rain, Fog (global Evasion/Blind), Rain of Fire/Blizzard (dodge to survive), Earth Quakes & Tornados, Poison Mist (Global DPS), etc.
Random Elevating Panels: A grid of elevating tiles that could trap you or provide a safe escape for you.