Mega Goodra
Type: Dragon -> Dragon/Poison
Ability: Sap Sipper/Hydration/Gooey -> Regenerator (I know it's everywhere, but come on, this is a slug dragon)
New Moves: Poison Jab
HP: 90 -> 90
Atk: 100 -> 120 (+20)
Def: 70 -> 120 (+50)
SpA: 110 -> 130 (+20)
SpD: 150 -> 180 (+30)
Spe: 80 -> 60 (-20)
Flavor Concept: Goodra's tail and back are now covered by a massive snail shell. But it's not getting Shell Armor
Competitive Concept: More like Greatdra, amirite? Let's just sit back for a moment and bask in this bulk. 90/120/180, with Regenerator to boot and an immunity to Toxic. Ah, that's the stuff.
The question here isn't what makes Megoodra so awesome, as it's so easily answered. Great mixed offensive stats and movepool, mixed with amazing defensive stats, mixed with Sap Sipper or Gooey to help when it switches in and Regenerator to make up for its lack of recovery, mixed with a typing that resists Fire, Water, Grass (x4), Electric, Bug, Fighting, and Poison, and neutralizes Fairy from the equation. Y'all can figure out what to do with this monster. Want to run mixed? Special wallbreaker? Curse? Dragon Tail? Hell, Goodra can even play with Counter before switching out and regaining health.
The question is how anyone can stop Goodra when it can tank even super-effective Ice, Ground, Dragon, and lolPsychic moves and switch out like it's nothing. Well, for one, Steel handily walls Goodra's STAB coverage, and crucially, Goodra has no special-boosting moves to make that non-STAB Fire Blast hit any harder. Of course, as mentioned, it does get Curse, but that brings us to another flaw: its newly-lowered speed requires it to take not one, but two hits from most attackers before hitting back. While it handily absorbs Toxic Spikes, Goodra still can't deal with rocks or spikes, which can neutralize its Regenerator advantage. And finally, and this is the biggest reason why I'm okay with letting this monster loose, we're in a meta with all sorts of scary new offensive threats. Goodra's here to remind everyone, in case they forgot, that defense can be scary too.