Well... I have a few creations xD (around 300 right now and growing bigger every day)
Something like:
Mega Butterfree 1: Bug/Psychic
HP: 60
Atk: 25 (-20)
Def: 80 (+30)
SpA: 120 (+30)
SpD: 110 (+30)
Spe: 100 (+30)
Ability: Tinted Lens
Added moves: Psyshock
Why this mega could be viable but not broken on OU?
Bug/Psychic while isn't an amazing typing neither at offense nor defense, it does have some nice perks that Butterfree can use. That typing helps Butterfree to not have a 4X SR weakness and with Tinted Lens, it can have two STABs that can deal neutral damage to almost every threat in OU! Also the increased bulk can help Butterfree to set up at least a Quiver Dance with relative ease and having the option of dealing with some counters using Sleep Powder is a nice plus while +1 fully invested 120 SpA can deal really nice damage against a few things.
However isn't going to be broken for sure... Excellent Neutral coverage isn't enough because Butterfree will need more than one or two boosts to get through bulky mons with neutral attacks, 60-80-110 while decent in the special side, usually doesn't use a recovery move unless you wanna use a Mono attacking Butterfree (it is usable but helpless against 4x bug resists like Heatran, Klefki, Togekiss, Talonflame and Skarmory). Finally while 100 is a better speed tier, its still slower than a lot of pokes in OU metagame, which will means that Butter should get damage or status before being able to setup!
Offensive Calcs:
+1 252 SpA Tinted Lens Butterfree Psychic vs. 252 HP / 0 SpD Heatran: 198-234 (51.2 - 60.6%) -- 89.8% chance to 2HKO after Leftovers recovery
+1 252+ SpA Tinted Lens Butterfree Psychic vs. 252 HP / 4 SpD Skarmory: 306-360 (91.6 - 107.7%) -- guaranteed OHKO after Stealth Rock
+1 252+ SpA Tinted Lens Butterfree Bug Buzz vs. 252 HP / 96+ SpD Clefable: 204-240 (51.7 - 60.9%) -- 95.3% chance to 2HKO after Leftovers recovery
+1 252 SpA Tinted Lens Butterfree Bug Buzz vs. 0 HP / 0 SpD Thundurus: 250-294 (83.6 - 98.3%) -- guaranteed OHKO after Stealth Rock
+1 252+ SpA Tinted Lens Butterfree Bug Buzz vs. 0 HP / 0 SpD Landorus-T: 276-324 (86.5 - 101.5%) -- 87.5% chance to OHKO after Stealth Rock
+1 252+ SpA Tinted Lens Butterfree Bug Buzz vs. 88 HP / 0 SpD Gyarados: 228-270 (64.5 - 76.4%) -- 12.5% chance to OHKO after Stealth Rock
Defensive Calcs:
252+ SpA Mega Alakazam Psyshock vs. 0 HP / 0 Def Butterfree: 217-256 (83.1 - 98%) -- guaranteed 2HKO
252 SpA Life Orb Gengar Shadow Ball vs. +1 0 HP / 4 SpD Butterfree: 211-250 (80.8 - 95.7%) -- guaranteed 2HKO
252+ SpA Life Orb Teravolt Kyurem-B Ice Beam vs. +1 0 HP / 4 SpD Butterfree: 122-146 (46.7 - 55.9%) -- 77.3% chance to 2HKO
252+ SpA Hoopa Unbound Dark Pulse vs. +1 0 HP / 4 SpD Butterfree: 218-258 (83.5 - 98.8%) -- guaranteed 2HKO
Anothe Butterfree creation is:
Mega Butterfree 2: Bug/Flying
HP: 60
Atk: 15 (-30)
Def: 80 (+30)
SpA: 130 (+40)
SpD: 100 (+20)
Spe: 110 (+40)
Ability: Compoundeyes
Added moves: Hurricane/Focus Blast
Why this mega could be viable but not broken on OU?
The problem that a lot of QD pokes have is the lack of coverage... With Hurricane and Focus Blast, Butterfree gets a suprising amount of coverage, that is at first glance plagued by a lack of accuracy. However Compoundeyes helps a bit transforming 70% accuracy into 91% accuracy, affecting also Sleep Powder!
130/110 offensive prowess (the same as Espeon and Latios) with a Speed and Special Attack boosting move can be pretty scary considering High powered moves like Focus Blast, Hurricane and Bug Buzz and all of them still have secondary effects.
However it still not be broken, because of its typing... With SR being anywhere on OU and with Gale Wings being a huge problem (also with a not so hot 60-80-100 bulk) it couldn't even get into OU but UU or something like that! But it still be troubling if its well used!
Offensive Calcs:
+1 252+ SpA Butterfree Hurricane vs. 4 HP / 0 SpD Thundurus-T: 179-211 (59.8 - 70.5%) -- guaranteed 2HKO after Leftovers recovery
+1 252+ SpA Butterfree Focus Blast vs. 248 HP / 192+ SpD Heatran: 314-370 (81.5 - 96.1%) -- 56.3% chance to OHKO after Stealth Rock
+2 252+ SpA Butterfree Focus Blast vs. 4 HP / 252 SpD Eviolite Chansey: 294-346 (45.7 - 53.8%) -- guaranteed 2HKO after Stealth Rock
+1 252+ SpA Butterfree Hurricane vs. 248 HP / 108 SpD Mandibuzz: 276-325 (65.2 - 76.8%) -- 18.8% chance to OHKO after Stealth Rock
+1 252 SpA Butterfree Hurricane vs. 252 HP / 84+ SpD Clefable: 246-289 (62.4 - 73.3%) -- guaranteed 2HKO after Leftovers recovery
Defensive Calcs:
252 SpA Thundurus-T Thunderbolt vs. +1 0 HP / 0 SpD Butterfree: 212-252 (81.2 - 96.5%) -- guaranteed 2HKO
252 Atk Life Orb Weavile Ice Shard vs. 0 HP / 0 Def Butterfree: 198-234 (75.8 - 89.6%) -- guaranteed 2HKO
0 Atk Mandibuzz Brave Bird vs. 0 HP / 0 Def Butterfree: 218-260 (83.5 - 99.6%) -- guaranteed 2HKO
0 SpA Heatran Lava Plume vs. +1 0 HP / 0 SpD Butterfree: 146-174 (55.9 - 66.6%) -- guaranteed 2HKO
252 SpA Pixilate Mega Gardevoir Hyper Voice vs. +1 0 HP / 0 SpD Butterfree: 153-181 (58.6 - 69.3%) -- guaranteed 2HKO
252+ SpA Teravolt Kyurem-B Ice Beam vs. +1 0 HP / 0 SpD Butterfree: 204-242 (78.1 - 92.7%) -- guaranteed 2HKO
252+ SpA Mega Alakazam Psyshock vs. 0 HP / 0 Def Butterfree: 217-256 (83.1 - 98%) -- guaranteed 2HKO
Why to change what we have right now?
Well, Bug/Psychic isn't the best typing with no immunities and weakness to common Dark, Fire and Flying Moves which makes it hard to switch in without receiving damage (Lugia and Dragonite both have better bulk or typing with immunities to switch in) and to keep multiscale intact. Also 60-80-90 bulk is poor for a mega and a multiscale user... Another not so hot thing is that 80 speed still misses a few things at +1(it gets outspeed by positive natured base 145 Spe, with no boosts that are actually pretty offensive megas like M-Alakazam, M-Aerodactyl and practically any scarfer) and its really underwhelming at +0 especially with that bulk...
The actual stat distribution reminds me of Beautifly, since it does have usable SpA but with lower speed, bad bulk and non attacking boosts which makes it almost unusable, while at least base 100 or 110 does have a better chance at setupping a QD and then sweep or just attacking against those pokes that can outspeed and destroy...
Whaddaya think guys? :3
Edit 1: since I don't like doubleposts I'm going to keep adding megavolutions! :3
Mega Venusaur Z: Grass/Steel
HP: 80
Atk: 122 (+40)
Def: 93 (+10)
SpA: 130 (+30)
SpD: 100 (+0)
Spe: 100 (+20)
Ability: Sap Sipper
Added moves: Heavy Slam, Flash Cannon
Why this mega could be viable but not broken on OU?
Well, Grass/Steel has been a really nice offensive and defensive type as Ferrothorn demonstrated! However Mega Venu Z is taking the opposing route going offensive with a nice but not amazing bulk to take some hits thanks to a huge array of resistances. Mega Venu Z can play mind games since Mega Venu XY is neutral to fire and resistant to fighting that are the two weaknesses of Mega Venu Z.
123/130/100 are nice attacking stats and can be evved mixed to hit a few things. Also, while 100 isn't a huge speed tier, its good enough for a poke with the ability to shrug a lot of attacks thanks to the coveted grass/steel typing!
Also, while its typing is a plus, its defensive stats are not as amazing as the typing... 80/93/100 while decent, it can still be less than optimal to avoid taking a lot of damage from fire and fighting attacks and it can't punish physical attackers as Ferrothorn because of its ability... Then, while Sap Sipper is nice to get an attacking boost, Grass attacks aren't as frequent on OU as they are on lower tiers and Venusaur already have ways to boost its attack and it would love more a way to boost its speed.
Edit 2
Mega Blastoise Z: Water
HP: 79
Atk: 103 (+20)
Def: 130 (+30)
SpA: 100 (+15)
SpD: 140 (+35)
Spe: 78 (+0)
Ability: Serene Grace
Added moves: No new Moves
Why this mega could be viable but not broken on OU?
Heck... this mega is almost the same as Suicune, without the ability to heal passively with Lefties or without the ability to Calm Mind... so, why would you use that? the answer is simple: Serene Grace boosting the chances on Burning enemies with Scald, paralyzing some others with Body Slam and then going flinching with Rock Slide (60%), Waterfall, Zen Headbutt or Dark Pulse (40%)... While this seems like a cheap option, Togekiss always demonstrated how effective it could be.
Finally this pokémon is far from broken because its bulk is nearly similar to Suicune (Which is UU ATM) and its even slower... even if Serene Grace can be pretty disgusting, still Electric Types can deal with Mega Blastoise Z since almost all of them are special and do not mind a Scald burn a lot.
Finally, Non invested attacks aren't going to deal with many stall teams that can still get the best of Mega Blastoise Z, since if it does run a lot of Serene Grace moves, it will get slowly broken down by its enemies because it doesn't have reliable recovery...
Edit 3
Mega Beedrill Z: Bug/Poison
HP: 65
Atk: 60 (-30)
Def: 40 (+0)
SpA: 145 (+100)
SpD: 80 (+0)
Spe: 105 (+30)
Ability: Speed Boost
Added moves: Bug Buzz
Why this mega could be viable but not broken on OU?
Mega Beedrill Z is the opposite of Mega Beedrill XY being special instead of physical... Bug Buzz/Sludge Bomb can hit hard coming from 145 SpA and while 105 speed is not stellar, it can outspeed a really huge bunch of the meta after a speed boost... Heck, you can even run Modest if you use Protect and still outrun a lot of pokes. Also, Beedrill has a really nice option in Baton Pass! Doing that, Beedrill can also support its team passing the speed boosts while dishing decent damage.
Another cool thing about being opposite of Beedrill XY is that you can play mind games about which mega you have, and people expecting a Protect and getting a sub will get hit through it by Bug Buzz. Another mind game is using Mega Bee Z as a quick defogger on an expected Protect, to further aiding the team.
However, this isn't going to be broken since it doesn't have any ways to boost its special attack and its coverage isn't exactly amazing. The worst part of mega Beedrill Z is just the bulk... Its bulk is just abysmal and every Aqua Jet, Ice Shard, Sucker Punch and Extreme Speed will get a huge chunk of health bypassing every single speed boost that it could get.