In the Japanese version, the Dark type is actually equivalent to "Evil" with Psychic being the equivalent of a "Light" type. This may be the only reason why the type has been solely under-represented in terms of Gym Leaders and Elite 4 members.
As a matter of fact, there's only been two primary users of the Dark type - Karen (Johto) and Sidney (Hoenn) and no Gym Leaders. Compare that to the other types...
(Elite 4 members in italic font)
Normal: Whitney (Johto), Norman (Hoenn)
Fire: Blaine (Kanto), Flannery (H), Flint (Sinnoh)
Water: Misty (K), Wallace (H), Juan (H - Emerald), Wake (S)
Electric: Surge (K), Wattson (H), Volkner (S)
Grass: Erika (K), Gardenia (S)
Ice: Pryce (J), Glacia (H), Candice (S)
Fighting: Bruno (K/J), Chuck (J), Brawly (H), Maylene (S)
Poison: Koga (K/J), Janine (K)
Ground: Giovanni (K), Bertha (S)
Flying: Falkner (J), Winona (H)
Psychic: Sabrina (K), Will (J), Liza/Tate (H), Lucian (S)
Bug: Bugsy (J), Aaron (S)
Rock: Brock (K), Roxanne (H), Roark (S)
Ghost: Agatha (K), Morty (J), Phoebe (H), Fantina (S)
Dragon: Lance (K), Clair (J), Drake (H)
Steel: Jasmine (J), Byron (S)
So in terms of underepresented types, we have Ground, Grass, Flying, Bug, Steel and Dark. Of those, Dark hasn't had a Gym Leader and one of the Elite 4 is quite frankly, a joke. And because of the apparent hatred for Sneasel and low levels of the Johto Elite 4, Karen can't use a full Dark team and has to use two Poisons (Vileplume and Gengar) to fill out her roster.
There's enough quality Darks already to allow for a full team - Umbreon, Houndoom, Spiritomb, Draipon and Tyranitar as the closer. Add in the inevitable newcomers a 5th gen would bring in with potential new type combos and this would be an absolutely vicious 8th gym.
And if they want to play up the "evil" aspect of the gym, I'd set up the 8th gym leader as someone who's got their trainers under some sort of mind control. Or if you prefer, More Than Mind Control. They don't have to be the leader of Team WhateverTheHell - we did that already, and it wouldn't have the same impact as when Giovanni shows up in Kanto. All they have to be is the ultimate manipulative bastard, give them a Dark Pulse TM and send them up to the Elite 4.