So I looked at Plusle and Minun's moves and they had some interesting things to work with...
Mega Plusle
Type: Electric
Ability: No Guard
Item: Pluslenite
Stats: 60/50/40/85/75/95 ==> 60/90/60/95/95/105 (0/+40/+20/+10/+20/+10)
So both Plusle and Minun both have a very interesting move that I think many people, myself included, totally forget that they had: Dynamic Punch. With No Guard and the increases in offenses, Plusle will be able to fire off 100% accurate Thunders, Dynamic Punches, and even Iron Tails, which will prove to be a great boon to it with the 100% confusion rate on Dynamic Punch. I didn't add very much speed, just so that it wouldn't be able to spam Dynamic Punch on everything, and while 105 is a great speed tier to be in in NU, it isn't among the fastest of the fast. With the increased bulk, it'll be able to survive a few more neutral hits, especially on the special side, but overall it isn't enough to make it shoot out of NU.
Mega Minun
Type: Electric
Ability: Simple
Item: Minunite
Stats: 60/40/50/75//80/95 ==> 80/60/70/75/130/85 (+20/+20/+20/0/+50/-10)
With increses in all of its defenses Mega Minun would be able to survive quite a few hits, and is only helped out in its survivability by the fact that it has access to Wish, though that isn't what it'll be using most of the time. Though its survivability is helped immensly, the main draw of this Mega is Simple combined with Nasty Plot, as it shoots Minun's Special Attack up to monstrous levels almost immediately. At maximum investment, with a positive nature, after a single Nasty Plot Mega Minun would reach 1092 Special Attack, something that no other Pokemon in NU can claim to accomplish with a single turn of setup. The downside to this Mega, barring the obvious lack of item, is the fact that its Speed has taken a hit, and while 85 isn't terrible, there are certainly many Pokemon who could take advantage of it. The other downside is that, even with a +4 Mega Minun you won't be able to one shot eveything in the tier, some of the Special walls still stand firm. Some calcs, keeping in mind that most people will run Timid due to the speed drop:
+4 252 SpA Minun Thunderbolt vs. 252 HP / 252+ SpD Dragalge: 96-113 (28.7 - 33.8%) -- 98.6% chance to 4HKO after Black Sludge recovery
+4 252 SpA Minun Signal Beam vs. 252 HP / 136+ SpD Eviolite Roselia: 107-126 (35.1 - 41.4%) -- guaranteed 3HKO
+4 252 SpA Minun Thunderbolt vs. 252 HP / 0+ SpD Regice: 151-178 (41.4 - 48.9%) -- guaranteed 3HKO
+4 252 SpA Minun Signal Beam vs. 252 HP / 40 SpD Uxie: 262-310 (74 - 87.5%) -- guaranteed 2HKO
It's able to manhandle these walls pretty well, but not OHKO them outright, and his defense is still quite sub-par, able to get destroyed by Archeops, Kangaskhan and the like with only a few blows. A very powerful Mega, but ultimately its lowered speed and still sub-par Defense hinders it and makes it so that it wouldn't shoot straight out of NU.