Mega Evolutions

I think they'd be something like a charge move.

Niantic must know from jut how absurd mega stats are they would more or less break the game and essentially devolve it into "do you have a mega mewtwo with shadow ball/mega rayquaza/primal groudon or kyogre" for legendaries and for nonlegendaries mega tyranitar/salemence/metagross/gyarados/Garchomp/Cd starters, if not, good luck because of how much they outstat other pokemon. That or mega's would need a MASSIVE nerf, looking at mega mewtwo Y's CP, its around 7k, compared to its slightly over 4k right now. This seems like it would be something thats had for maybe a few seconds/attack then revert back to normal, or they be heavily nerfed

If they do implement mega's as standalone pokemon it would likely need to be sizably nerfed statwise, or have such a ludicrous cost to get to make it not worth it, maybe for the legendaries be like 400+ candy

Personally, i'd rather they never implement megas, they were a terrible idea for the main series, and most of them look...dumb. In GO, they would probably make some tier 4's and possibly tier 5's soloable, theres already a few of them that are soloable, though require maxed mon at near perfect iv's in weather boost on specific targets. (Tyranitar, rayquaza) mega's likely would make them possible to solo without weather boost.

T4 raids are already entering the realm of solo-ability thanks to the effectiveness buff. I've seen multiple Absol solos, I've seen IT/FB Tyranitar solo'ed, and I've seen Rhydon solo'ed.

I would assume megas would follow the same rules as in the main series; a pre-existing Pokemon mega evolves mid-battle, only one per fight, if it faints it cannot re-mega evolve that same fight. I think they'll definitely make T4 solos a bigger possibility for most, and potentially some T5's as well. Assuming they don't change raid fundamentals or buff the bosses to counter-balance the fight.

I would just be interested how they would be implemented. Ive never played X and Y but from what i understand you need 2 items and it only lasts till the end of the battle. The mon-specific item could be substituted for candy but how much? Blowing 100 candy for a single battle seems too extravagant for me. But having something with that stats permanently may very well break the game as we know it. If they do make it a separate mon they should at least reroll the moves.

Something that we can be sure of is that if Niantic implements them they'll try to make a profit off it. Whether it's temporary or permanent in Pogo, they'll for sure require lots of candy or consumable items that only drop from raids or have some other way of incentivizing coin expenditure. Megas are so powerful that they can be used to generate a lot of excitement in the playerbase but Niantic also probably wants to limit access to the best ones for powerlevel reasons. Battling against Mega evolved raids would be pretty hype I think.

How they handle PvP with them would be interesting. Allowing one mega per battle in master league could spice things up with additional strategic options as long as it doesn't turn into a battle of who can afford the resources to mega evolve every battle.

Before effectiveness change, Shuckle would have been soloable only with Primal Origin Pulse Kyogre in rain, so that's one case at least where the unsoloable becomes soloable thanks to them.

The Niantic we know might treat megas as forms. The most likely way of introducing them would be as separate raid bosses, so we would have to catch the mega form rather than evolving its normal form. Does it sound odd? Of course, but this is the current trend already used for several forms (castform, shedinja, giratina, deoxys...).

We can only hope that Niantic deviates from their current trend and handle mega evolutions in a better way. So far they've only shown lack of imagination with the different available forms. Primal forms will be most likely treated as separate raid bosses as well. That might be the way of introducing precipice blades, etc.

The logical way to implement mega evolutions would be similar to adding a second charge move. Pay a massive amount of candies and possibly stardust to unlock the mega evolution for a pokemon. Once per battle you would be able to press a button to activate a mega evolution for one of your pokemon, giving it a buff for the rest of the fight. It could either be usable as soon as you send out the pokemon, or require building up energy first.

I could possibly see them treating mega pokemon as different forms. I feel like they would be treated as mythical pokemon (if they aren't already are, ex Tyranitar, Charizard, ect) I can't see them being allowed to be placed on gyms and I feel like they might put a cap on how many a player could have just to avoid people's teams becoming insanely overpowered. I also see them getting nerfed heavily. I don't think that they'd have mega pokemon be insanely strong because it would essentially break the game.

I think a good idea would be that any situation that rewards candy should have a chance at dropping an "-ite" instead.
Catch a Pidgey, and you might score a Pidgeotite instead of Pidgey candy. Hatch a Mudkip and you might get a Swampertite. Raid an Absol, chance for Absolite if caught.

I'd also love it if you could stumble on Mega Stones while exploring the map and simply tap to collect. I would definitely get out and explore more.

Personally, I would make them very hard to get. But not impossible. And I would use this as a way to introduce a way to TRAIN a Pokemon. In the game there is nothing that lets you Train them - you spend resources to Evolve or Power them up, but you can literally catch a Level 1 Pokemon and power it up to Level 40 and evolve it to the max level, buy a second Charged Move for it but never use it. This has always rankled me, but Mega-Evolution could fix that to a small degree.

So how I would implement it: (not a complete plan, just an outline)

1) Unlocked at Trainer Level 40 (+, if the level cap is ever increased)
2) Useable only on Pokémon Pokémon at Level 40
3) Proper Mega-Evolution Item, tied to a particular Pokémon
-I would change things up a bit here from the games/anime and have the Mega-Evolution (ME) items be done by Typing, so there would be a Psychic ME Item, a Dark ME Item, a Fighting ME Item, etc.
4) Battles Won with this Pokémon AFTER gaining the proper ME Item: 100+
-If there is a way to track this, I might allow for Soloing a Raid Boss with this Pokémon to count as multiple victories.
-I might break it down to include a certain number of victories from different sources, like 33 Gym Victories, 33 Raid Victories, 33 PvP Victories, 1 Gym Defense Victory (well... okay, that might be nigh-impossible for ANYONE to get without some form of assistance from another player who intentionally loses a gym battle against your Pokémon, so maybe not)
-I might require a certain number of KM be walked, or Candies to be gained via Walking as a Buddy.
-If they ever allow for Legendary or Mythical Pokémon to be put into Gyms, I would consider a certain number of hours as a Gym Defender be part of the equation. Honestly, I'd prefer that Legendary and Mythical Pokémon NOT be able to Mega-Evolve, but they already exist in Pokémon, so... meh.

USING the Mega Evolution:
1) Every Mega Evolved Pokemon would gain a set amount of HP, then their stat changes would be based on the stat changes from the main game...but the Speed would be a problem. For Mega Pokemon with high Speed, I'd roll that into the defense somewhat (ie: Mega Beedrill has Speed +70 but +0 to any Defense, so I'd probably make it +40 Defense) but then deal with the Speed in #2 - Moveset.
2) The Mega Evolved Pokémon would have access to a different moveset, which would be set by the Trainer. Because of the lack of a "Speed" attribute, I would allow Mega-Evolved Pokemon access to moves with lower Cool-Down times (Like Bubble Beam vs Aqua Jet) even if they don't normally have access to them, as a way to try to simulate having a faster Speed Attribute. I might extend this to those who get a sizeable "Special Attack" boost by allowing them access to other moves that are more powerful (like Lick vs Hex vs Shadow Claw), to simulate that boost, rather than just giving Mega Gengar: +40 Attack, for example.
3) I would have a Time Limit on it - like 60 Seconds - before the Pokémon faints and has to be revived.
-If a Mega-Evolved Pokémon is put in a Gym, then the Pokémon uses its Mega Evolution in the gym battle. Which means that the Attacker will "win" the battle as the Defender will faint before the timer runs out, but it may do quite a bit more damage to the Attacker.

Other aspects:
-I would likely have the ME Items being like Mythical Pokémon - you can only have 1 of any single type. So 1 Dark ME Item, 1 Psychic ME item, 1 Steel ME item, etc.
-I would consider being able to transfer the ME Item to another Pokémon while the first is being Trained. (I'd look for a way to keep the progress on the First Pokémon, tho - one of the little things that rankles me about PoGo is how Buddy Distances get lost when you switch buddies. Yet the total distance is still kept! While I understand why this is the case, it's still something that rankles me)
-I would consider buying the second Charged Move to be part of Training in some way, though I don't know how that would work.
-I would likely have the ME items be drops from 7 Day Streaks or SOLO Raids (though I'm not sure if the game can currently tell when you've Solo'd a Raid or not; maybe introduce a "Solo" option for a Raid, along with "Private Group"?). If it couldn't be from Solo raids, then make them quite rare drops from any Raid or 7 Day Streak.
--Drops from Raids would be from the Typing of the Raid Boss. So if you beat a Mawile, you could get a Fairy or Steel ME Item. If you beat a Shinx, it would only be an Electric ME item.
-Assuming there is ever an increase to the Level cap, I would make it so that only Pokemon with Mega Evolutions can be raised above Level 40. And raising levels would be dependent on use ONLY - every battle the Mega Pokemon fights in (whether Mega-evolved or not) raises its level a little bit. So the more you use it, the stronger it becomes.
--Would likely use the Gym XP model for raising the level of Mega Pokemon, so beating a CP 100 Pokemon in a Gym Battle wouldn't be as useful as beating a CP 3000 Pokemon, but would still count, and beating Raid Bosses would still be the best way to do it.