I've been hoping for one since Diamond and Pearl. It could happen, but every type has a shot at having its gym. Usually, dark types are put into an Elite 4 slot, covering 3 of the 4 generations it has existed.
Private Mod Note
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"On my business card, I am a corporate president. In my mind, I am a game developer. But in my heart, I am a gamer" -Satoru Iwata
Puzzle- find switches in a dark, light-less maze as you fight trainers to get to them. When you found all the switches, a chandelier will drop down and reveal light to get to the gym leader.
Battle- the gym leader will have these pokemon: Aykey (normal and dark type evolution of this gen's Rattata-clone), Bisharp, and Wherewon(new werewolf-like dark type). Once you've defeated his pokemon, you will get the gym badge as usual, but he'll now leave his gym to head to the post-game area.
Take note that this will be the last gym in the game.