Another idea for a Dark Gym: each trainer breaks the rules in some way. Two challenge you at a time while you're only allowed one pokemon, one starts with Toxic Spikes on the battlefield, one has seven pokemon with him, one starts with your pokemon confused, one uses pokemon that are anything but Dark types and nicknames them as other mons entirely, the Leader has a Zoroark that has different moves in his Illusion form, which is not a pokemon in his party...

Stuff like that. You get a riddle before each battle that clues you in on the cheating tactic up next.