hihi I'm Ishy-Hime, a.k.a. Ishtaria! Since we are talking about possible suspects, bans, and such, I thought I would share my ideas about it.
These reasonings are and will be from a Dragon User's perspective, so there's that!
We'll start things off with a "pressing issue", a.k.a. Mega Altaria. To keep these short I'll just do one mon at a time.
Mega Altaria: First things first, Mega Altaria (imo) is an answer to the current metagame's condition. But it still has multiple counters already, like Sash Bisharp, Genesect, Scarf Magnezone, Zard X (even though I don't have issues with it, it still is a counter), Mega Gardevoir, Mega Diancie, Heatran, etc., so banning it would be somewhat unnecessary. Now it can cause unfair matchups, but it also can cause hindering matchups for the user as well, which I believe people forget. Most Mega Alt sets are the usual DD/Roost/Return or Frustration/Refresh or Heal Bell (player's preference tbh) and as for spreads, it's either King DDD (Defensive Dragon Dance) or 64HP/192Atk/252Spe. These spreads/sets are based off of the majority of Mega Altarias. I do understand that some types struggle against Mega Alt, which can cause centralization to stop it, and I do agree that Mega Alt should be suspected, but it shouldn't be a top priority. As for matchups go (I am not counting the Dragon Matchup, since that is stupid tbh, you have scarfs and banded Dragonite to handle Mega Alt and Mega Alt beats Mega Alt so no point. Plus I'm only doing the 3 matchups I feel Mega Alt simply takes over in):
Electric: So it's not that much of a secret that Electric struggles against Dragon (Fire also struggles with Dragon, but not Mega Altaria), so why is Mega Alt a problem for this matchup? Solely from the extra bulk it obtains from Mega Evolution and the ability to Dragon Dance is what makes the matchup more of an issue, but there are ways around it. Mega Alt can cause centralization with Electric, but this centralization could possibly help the type to progress in the metagame. Oh and these calcs are BEFORE a Dragon Dance AND without Rocks up.
252 SpA Magnezone Flash Cannon vs. 64 HP / 0 SpD Mega Altaria: 254-300 (82.7 - 97.7%) -- guaranteed 2HKO
252 SpA Life Orb Thundurus Hidden Power Ice vs. 64 HP / 0 SpD Mega Altaria: 161-190 (52.4 - 61.8%) -- guaranteed 2HKO
192+ Atk Pixilate Mega Altaria Return vs. 248 HP / 216+ Def Rotom-W: 144-169 (47.5 - 55.7%) -- 21.5% chance to 2HKO after Leftovers recovery
Okay so here are some calcs for Electric v Mega Alt on Dragon. The issue with the Thundurus calc is, Mega Alt's Return does at least 90.3% so the Thundurus Calc is to weaken Mega Altaria for later on. Bulky Rotom-W can tank a return, burn it, and Pain Split the Mega Alt. Most Mega Altaria's don't run Earthquake from what I have seen, so your Magnezone should take a few Returns from that Mega Alt.
Fighting: Okay so Fighting can easily get swept by a Mega Alt AFTER two Dragon Dances and Rocks ofc. Why two? Scarf Terrakion, Scarf Icy-Wind Keldeo, and Sash/Scarf Gunk Shot Infernape. Although Gunk Shot Infernape isn't as common, it still is a counter to Mega Altaria. Mega Medicham can counter Mega Alt to an extend with Fake Out/Bullet Punch. Again, Fighting does have troubles with Dragon, Mega Alt just helps Dragon out. Calcs stay the same, before DD and no Rocks. Mega Medicham's work on Mega Altaria:
252+ Atk Pure Power Mega Medicham Fake Out vs. 64 HP / 0 Def Mega Altaria: 74-88 (24.1 - 28.6%) -- 98.3% chance to 4HKO
252+ Atk Pure Power Mega Medicham Bullet Punch vs. 64 HP / 0 Def Mega Altaria: 148-176 (48.2 - 57.3%) -- 91.8% chance to 2HKO
Although this combo does not knock out Mega Altaria, you can still work around it, by managing two fake outs and a bullet punch and bop there goes Mega Alt. This is at risk for sacking one mon to take out the biggest threat for Fighting. Now Scarf Keldeo isn't going to OHKO Mega Altaria with Icy Wind, but it slows down and weakens Mega Altaria enough for Mega Medicham to clean or even your Gunk Shot Infernape. If Infernape is sashed, then it can take Mega Altaria out before it gets knocked out.
252 Atk Infernape Gunk Shot vs. 64 HP / 0 Def Mega Altaria: 206-244 (67.1 - 79.4%) -- guaranteed 2HKO
Although this isn't common, centralization will occur until a type has a definitive counter to a certain pokemon.
Dark: Don't get me wrong when I said Mega Alt quite easily can sweep Dark, but Dark does have counters for it. There's Weavile, Bisharp, and Drapion. So in XY, Dragon had issues with Dark with Greninja and Sableye. Now that the tables are turned, Mega Alt with at least 2 DDs can sweep Dark without possibly getting knocked out. Weavile is seeing a lot more play in Dark which hinders a Mega Altaria that hasn't set-up. Even Sash Weavile (without rocks) can bop a Mega Alt. Calc Time! No Rocks, No Dragon Dances! First up, LO Weavile:
252 Atk Life Orb Weavile Ice Shard vs. 64 HP / 0 Def Mega Altaria: 151-179 (49.1 - 58.3%) -- 98.4% chance to 2HKO
252 Atk Life Orb Weavile Icicle Crash vs. 64 HP / 0 Def Mega Altaria: 315-374 (102.6 - 121.8%) -- guaranteed OHKO
So saying Dark has nothing for Mega Altaria is a lie. Weavile with Speed and Attack and stop Dragon easily which is why Mega Alt can't stop Dark as easy as people think! Bisharp!
252+ Atk Bisharp Iron Head vs. 64 HP / 0 Def Mega Altaria: 258-306 (84 - 99.6%) -- guaranteed 2HKO
192+ Atk Pixilate Mega Altaria Return vs. 4 HP / 0 Def Bisharp: 204-240 (75 - 88.2%) -- guaranteed 2HKO
Although in this circumstance Mega Alt outspeeds, Bisharp does have access to priority moves like Sucker Punch and, if predicting a switch, Pursuit. Bisharp is one of Altaria's biggest concerns that it can handle. Now time for the last calc for Mega Altaria, the somewhat common calcs of Drapion!
252 Atk Drapion Poison Jab vs. 64 HP / 0 Def Mega Altaria: 188-224 (61.2 - 72.9%) -- guaranteed 2HKO
192+ Atk Pixilate Mega Altaria Return vs. 0 HP / 4 Def Drapion: 186-220 (66.1 - 78.2%) -- guaranteed 2HKO
Drapion wins in this circumstance due to it's speed, and before you claim that EQ can OHKO, it actually only does 0.8% more. The Calcs that I have done, simply prove that even unfair matchups can stop Mega Altaria. The calcs only prove that Mega Alt isn't broken, it's just really good for the types that can use it.
Dragon: Suspect Mega Altaria if needed
Flying: Leave Mega Alt as is
If you read all of this, Thanks for hearing my reasonings for Mega Altaria staying in Monotype!
tl;dr Mega Altaria is a problem for some types and there are ways around Mega Alt, suspecting it on Dragon should only be if necessary and Flying doesn't need nor uses Mega Altaria as much as Mega Charizard X, so banning/suspecting it on Flying would be pointless.