I've always thought I'd be Dark or Ghost! I mostly considered Dark since there wasn't a single Dark gym leader until Gen 8, but Ghost is probably more accurate. I'll do teams for both, though!
Dark
Umbreon - Sucker Punch, Dark Pulse, Moonlight, and Psychic
Zoroark - Snarl, Night Slash, Retaliate, Protect
Absol - Slash, Swords Dance, Future Sight, Perish Song
Mightyena - Crunch, Yawn, Play Rough, Payback
Houndoom - Beat up, Fire Fang, Flamethrower, Embargo
Thievul - Parting Shot, Fake Tears, Assurance, Acrobatics
Honorable Mentions: Liepard, Honchkrow
Reward: Snarl TM, Nightshade Badge
Gym Description: It looks like a dark cobblestone street just barely lit by fire lantern streetlights. You can walk straight on through, but no one will be there until you veer off into the dark alleyways and fight the trainers.
Ghost
Mismagius - Phantom Force, Power Gem, Rain Dance, Thunder
Polteageist - Memento, Wonder Room, Shadow Ball, Giga Drain
Cursola - Hex, Curse, Will-O-Wisp, Brine
Banette (Mega) - Toxic, Hidden Power (Fighting), Shadow Claw, Dazzling Gleam
Froslass - Draining Kiss, Aurora Veil, Blizzard, Avalanche
Gourgeist - Trick or Treat, Leech Seed, Shadow Ball, Brutal Swing
Honorable Mentions: Chandelure, Drifblim
Reward: Shadow Ball TM, Wisteria Badge
Gym Description: The gym is a soft grass lawn with a wisteria archway. Going through, you are asked questions and are given small insights based on it. Depending on the insight, you fight one of two trainers per question: attack and special attack, defense and special defense, and speed and hp. That's a lil half-baked but that's the idea.
I was thinking rematch teams for this... During the main story I feel pretty comfortable saying I'd be the sixth gym leader!