I like this as an opportunity to make some not-so-strong Pokemon more viable. Take for instance Mightyena, a Pokemon that I really enjoy from a design standpoint, but lacks power.
Mega Mightyena
Normal Stats: 70/90/70/60/60/70
Mega Stats: 70/135/80/60/70/105
Mega Ability: Moxie
Considering Mightyena's low BST, I didn't feel too bad putting lots of points into Attack and Speed. 105 Base Speed is a fantastic speed tier, but not as fast as some of OU's biggest threats. 135 Attack without a boosting item is certainly balanced in my opinion, and means you won't have to forfeit if it gets a Moxie boost. Mightyena doesn't have a great movepool, but the combination of Sucker Punch/Crunch and Play Rough is mostly unresisted (Klefki/Mawile being the only ones). I would, however, give Mightyena access to Pursuit, too give Mega Mightyena a niche as a trapper which turns into a wall breaker with a Moxie boost. Here are some calcs too show the kind of strength Mightyena could have.
+1 252+ Atk Mightyena Sucker Punch vs. 252 HP / 0 Def Aegislash-Shield: 312-368 (96.2 - 113.5%) -- 81.3% chance to OHKO
+1 252 Atk Mightyena Sucker Punch vs. 252 HP / 0 Def Aegislash-Shield: 284-336 (87.6 - 103.7%) -- 25% chance to OHKO
Aegislash is obviously a big threat in this meta, but Mega Mightyena would have a chance to OHKO after scoring a trap kill with Pursuit and then using Sucker Punch til Aegislash attacked. However this chance is much higher if Mightyena is running Adamant nature, but that would cause it to lose out on outspeeding Max Speed Base 100s.
+1 252 Atk Mightyena Play Rough vs. 4 HP / 0 Def Garchomp: 316-372 (88.2 - 103.9%) -- 25% chance to OHKO
Garchomp is another big threat in the meta, but given a Moxie boost and Jolly nature, Mightyena could outspeed non-Scarf, possibly OHKO Garchomp, and can survive any non-Banded hit from it if it doesn't.
+1 252+ Atk Mightyena Sucker Punch vs. 4 HP / 0 Def Mega Charizard Y: 271-321 (90.9 - 107.7%) -- 50% chance to OHKO
+1 252 Atk Mightyena Sucker Punch vs. 4 HP / 0 Def Mega Charizard Y: 247-292 (82.8 - 97.9%) -- guaranteed 2HKO
Mega Charizard Y is extremely prominent in the meta, and is a scary Poke. These calcs again show the trade offs between Adamant and Jolly nature. Adamant (again assuming Moxie trap) can possibly OHKO with Sucker Punch, and thanks to priority, can hit before Char Y. With Jolly, it can outspeed, but not OHKO, and will die to a Fire Blast from Char Y.
Personally, I would run an Adamant nature, with EVs in Attack and Speed, Intimidate in base form (take less damage when you come in to Mega Evolve) and a Move Set of
-Sucker Punch
-Pursuit
-Play Rough
-Substitute
The first moves are pretty self explanatory (Priority STAB, trapping move, coverage), but Substitute would allow for an easier time against most supports and walls, the chance to counterplay an opponent expecting Sucker Punch and using a status move, and a way to avoid the ever-present Will-O-Wisp.