Name: Mismagius
New Stats: 80/60/70/145/120/115 (590)
New Typing: Ghost
Movepool Addition(s): +Recover, +Focus Blast, +Healing Wish
New Abilities: Levitate/Magician/Illusion
Justification: I'd give it Psychic-type for the magic theme, but since that makes the Dark-type weakness even worse, that's a no go. The abilities, on the other hand, are fully magic-related, while the added moves are mostly generic-ish moves.
Niche in OU: While it may lack in a secondary STAB or high power moves, Mismagius now possesses a gigantic 145 SpA, which becomes even more of a threat with Nasty Plot. A Timid LO set at +2 outright breaks everything save for Clefable in at most 2 hits, and at 115 Speed a Scarf set can be devastating as well. Calm Mind, on he other hand, can be used with Moonlight for further survivability, though physical moves would be a huge concern for you.
Name: Dusknoir
New Stats: 95/110/140/65/140/40 (590)
New Typing: Ghost
Movepool Addition(s): +Drain Punch, +Dark Void, +Hammer Arm, +Signal Beam
New Abilities: Pressure/Iron Fist/Intimidate
Justification: With it being this fat, I think Dusknoir deserves to have a much, much higher HP than it does atm. Iron Fist buffs just about every important move in Dusknoir's hands bar Earthquake, Drain Punch because I'm more than certain that it being a ghost means it can suck life forces. Dark Void is a legendary move, yes, but
OM room did it so I'm doing it too. Signal Beam is more of a flavor addition, as according to its pokedex description.
Niche in OU: Having a completely ridiculous bulk and a serviceable 110 Atk, Dusknoir can now run Assault Vest as an alternative to its Band set. Now that Shadow Punch is actually somewhat usable, it can be much more powerful in terms of damage dealt with STAB, not having to rely on Shadow Sneak only. Even though it lacks reliable recovery, Drain Punch can decimate most Dark-types switching in, while healing Dusknoir of some of its damage, while Hammer Arm can be an option for even more damage. For a defensive/support set, Dark Void and Intimidate can be a tremendous help, being a tool to cripple one of the opponent's team and one that can ease switch-ins to physical moves and forces switches respectively.
Name: Golurk
New Stats: 100/140/95/65/120/70 (590)
New Typing: Ghost/Ground
Movepool Addition(s): +Circle Throw, +Bulk Up, +Explosion, +Zap Cannon
New Abilities: Iron Fist/Solid Rock/No Guard
Justification: Solid Rock, while mostly a competitive addition, works due to its nature as a clay statue. Explosion references its pokedex entry mentioning its energy going out of control when the seal is removed, and Zap Cannon... Just, look at Pokepark 2...
Niche in OU: Compared to Dusknoir, this Golurk is more offensive oriented, with a sky-high Atk and decent 70 Speed for a tank. Solid Rock furthers its tanking capabilites, helping it in setting up Rock Polish/Bulk Up. Stealth Rock sets are viable of course, with a new tool in Explosion to leave with a boom after you've secured your rocks and Golurk is about to faint. It can also use Circle Throw to phaze opponents after setting up hazards, though without STAB... Zap Cannon? Oh I dunno, use it with No Guard or something to specifically screw over Skarmory?
Name: Drifblim
New Stats: 150/90/75/100/85/90 (590)
New Typing: Ghost/Flying
Movepool Addition(s): +Roost, +Hyper Voice, +Autotomize, +Scald
New Abilities: Aerilate/Unburden/Flare Boost
Justification: You know, air balloons' entire schtick is to float in the air. Aerilate sounds pretty fitting to me. Scald because steam balloons are a thing, and Autotomize not only symbolizes a hot air balloon leaving the loads behind, but also for this below...
Niche in OU:
Aerilate Explosion. Yeah, you heard that right, we're doing this now. Not taking up a mega slot like M-Glalie, you can basically slap a Scarf or a Sash and Autotomize on it and watch things die. Of course, the other two abilities are completely usable, Unburden providing speed for Rest or Acrobatics sets, and Flare Boost as a power house.