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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Reversion. A place like a coliseum, but there are portals strewn about. If you want into one, you go the the upside down version of the planet. The upside down version will be surreal, filled with never ending corridors, places where gravity has no affect, and dangerous creatures lurk It will be a good place for speedy, yet interesting fights