Except that mega gengar's off-limits if he sticks to the default smogon rules.
I did a mono-ghost run in White 2, trading a Golett egg from my copy of White and eventually catching a Jellicent, Drifblim, Banette, Chandelure and Cofagrigus, and beat the league once with it. It was the first time I did a monotype run as my first run of a pokémon game, and it was certainly interesting.
You also can't forget Dusknoir, Mismagius, Trevenant and the pumpkin, on top of Golurk, Drifblim, Cofagrigus and the ghosts other have mentioned before. All in all, I'd say ghosts have some decent variety in both types and playstyles.
I've just finished breeding a Diglett for my regular team after lots of on-and-off breeding, and that makes the third pokémon I've got. Now I should probably look into countering ice types and adding some tanks and better damage-dealers, since Dugtrio, Jumpluff and Crobat are all weak to ice on top of sporting below-average defenses.