Let me try my hand at this:
Name: Mega Stakataka
Ability: Stamina (4)
Typing:
Stats: 83/149/229/83/113/13 (+22 HP, +8 Atk, +22 Def, +12 SpD,
BST 670)
New Moves: Meteor Mash, Barrier
Overview: Stakataka functions in UU as an incredibly bulky trick room user, and since Trick Room isn't very prevalent in OU, I think it would be a good idea to have a good user of it. An increase of 20 base points in each of its defenses accounts for its poor defensive typing, lack of reliable recovery, and inability to hold a defensive item. Stamina increases its already incredible Defense to astronomical levels, while Barrier allows it to quickly boost up should the need arise. Stakataka already has strong STAB and coverage in Gyro Ball, Stone Edge, and Earthquake, so the addition of Meteor Mash just acts as a fallback. Steel-types such as Ferrothorn, Skarmory, and Celesteela all wall Mega Stakataka, with Ferrothorn notably able to stall it out with Leech Seed and Protect. Priority users (especially Mach Punch users) and bulky special attackers also threaten to severely cripple Mega Stakataka. However, even without reliable recovery, Mega Stakataka is hard to take down in one hit, and many Pokémon are OHKO'ed by it.
Name: Mega Nagandel
Ability: Dragon-ate (4)
Typing:
Stats: 73/139/73/149/73/133 (+66 Atk, +22 SpA, +12 Spe,
BST 640)
New Moves: Work Up, Earthquake
Overview: Although Nagandel is a mediocre Pokémon in Ubers (C+ on the Viability Ranking), I still wanted to make its Mega reasonably balanced for the Ubers metagame. Mega Nagandel is, quite possibly, the best mixed attacker in Ubers. Although its stats are not that impressive, especially because it cannot hold an offensive item, Work Up allows it to boost both of its attacking sets. Mega Nagandel can either use its physical or special STABs, furthering its viability. Earthquake gives it coverage against Primal Groupon and Necrozma-DM, who would otherwise wall it, and is used more effectively on the Mega due to normal Nagandel’s lower attack stat.
A 133 base Speed allows it to outspeed Marshadow and Mega Gengar. However, Mega Nagandel does nothing to improve on Nagandel’s terrible bulk, leaving it vulnerable to Choice Scarf and priority users such as Xerneas, Yveltal, and Arceus-Normal.
Name: Mega Blacephalon
Ability: Turboblaze (3.5)
Typing:
Stats: 73/127/73/181/107/127 (+20 HP, +20 Def, +30 SpA, +10 SpD +20 Spe,
BST 670)
New Moves: Moongeist Beam, Recover
Overview: Acting as a combination of the Choice Scarf and Specs set, Blacephalon gets a decent increase in both Speed and Special Attack. However, it is now finally able to switch moves! This means that Blacephalon is able to abuse its high power STABs, including Fire Blast and the newly acquired Moongeist Beam. It’s increased bulk also gives it more staying power in conjunction with Recover, letting it have more switch-in opportunities. Finally, a better speed tier let’s it overcome previous revenge killers such as slower Tapu Koko, Alakazam, and Tornados-Therian. Blacephalon still has poor coverage, and can be either walled or taken advantages by Pokémon such as Tyranitar and Greninja. Blacephalon also retains poor physical bulk and several common weaknesses, making it easy to take out with priority.
Edit: Fixed Stats. Now they should all be +100. Thanks!