Mega Golurk
Type: Ground/Ghost -> Ground/Ghost
Ability: Iron Fist/Klutz/No Guard-> Gravity Pull (Gravity is in effect while the user is on the field, grounding foes and decreasing evasion of all mons by 2 stages; Fly, Sky Drop, Bounce, Jump Kick, Hi Jump Kick, Splash, Telekinesis, and Magnet Rise cannot be used)
New Moves: Bulk Up, Shadow Sneak, Mach Punch
HP: 89 -> 89
Atk: 124 -> 169 (+45)
Def: 80 -> 105 (+25)
SpA: 55 -> 55 (+0)
SpD: 80 -> 110 (+30)
Spe: 55 -> 55 (+0)
Flavor Concept: Golurk's bandage comes loose, unleashing a spectral aura around its armor.
Competitive Concept: Oh, if only Golurk was Ghost/Ground instead of Ground/Ghost, we could make it Ghost/Steel, or Ghost/Fighting. But no dual-type Pokemon has ever changed its primary type through evolution, barring birds to maintain Flying a a secondary type, so as I already said, our Ground/Ghost golem stays.
But that's hardly a bad thing. Using its phantasmal powers and its affinity to the earth, gravity increases when Mega Golurk comes to play. Not only does its Ground STAB now hit all foes regardless of Flying type or Levitate, but the evasion drop acts as a pseudo-No Guard, allowing our golem to keep swinging with Dynamic Punch and Stone Edge. Its STAB Ghost/Ground coverage under Gravity is nigh-unstoppable, resisted only by Normal/Grass, Grass/Dark and the nonexistent Normal/Bug and Bug/Dark (barring new Megas of ours).
Improved bulk and an immunity to Normal, Fighting, and Electric help make Golurk a pretty great tank. Further assisting is the newly-added Bulk Up (unfortunately Curse doesn't work the way it does on non-Ghosts); while an all-out attacker set could do fine, Golurk can also invest entirely in its HP and superior Special Defense for a Bulk Up set, with new priority tools Shadow Sneak or Mach Punch for speed, Drain Punch for recovery, and powerful Earthquake STAB. Or, Golurk can maintain its niche as a spinblocking Rapid Spin blocker, using STAB Shadow Sneak off its incredible base attack to keep the Lati defoggers at bay.
But all is not well for our valiant automaton. Gravity hits both ways, and Golurk's weakness to the likes of Hydro Pump and Will o Wisp is compounded by their now-perfect accuracy against it. Scald is also still a hassle, and Golurk remains weak to Pursuit and Knock Off (even if it doesn't knock off its stone) as well as the common Ghost type coverage it so benefits from offensively. Still, though, with constant Gravity at its back and a newly-optimized movepool, Golurk has a new lease on the afterlife.