Well, as I type this. I am reworking one of my least viable characters into an actually viable arena combatant. 

His name is Richter, and he's one of my favorite characters but he was also ridiculously broken previously. Not impossibly strong or fast, for an arena combatant anyway, he was just unkillable because of what he was. (A swarming mass of smaller entities that conglomerated into a single hive mind.)