Instead of coming up with potentially boring Arena ideas myself, I decided that the combatants should come up with arenas themselves. Just give me the general setting for an Arena then a few thing you would want to see or would find cool to add to said Arena.
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A crystal world. A huge map layed out like a valley but made purely of crystal gemstones. Even the river running through the middle is a crystalline liquid that, should a combatant fall in, will turn them into a crystal structure that will lay in the river for eternity