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Figured since there wasn't a Dark-type gym, I'd give this a try.
Gym leader gimmick would be akin to a detective of sorts, who is looking to ascertain the whereabouts of the region's evil organization. Challenge would be to defeat trainers littered around the Gym City's harder to reach spots and piece together the clues to find where the Leader is located. His partner is a Sandile/Krokorok/Krookodile who acts as his muscle. He either uses a Mega Houndoom or Mega Sharpedo in his rematch depending on game version. (Lighter color = Houndoom, Darker = Sharpedo)
Victory yields the Noir badge and a Dark-type TM. Plot significance could be he nails down the evil team's hideout and sends you to the next part of your journey.