Mega Stones in ORAS PH
Disclaimer: 99% of this post is purely speculation on my part, and I made this mostly for fun. Please do not use anything below S- in this post unless you are cracked at the game and
really know what you are doing, and even then,
surely there are better uses for that teamslot.
I’ve been thinking for a little while now on the available mega stones in ORAS PH, and I came up with a VR of sorts that includes my thoughts on what could potentially be done with each one. If a stone is not ranked, I probably was not able to find a “genuine” niche for it.
Onto to the VR…
S: Established

Gyaradosite (Mold Breaker)
I don’t think I need to really explain this one, but I will anyways so that way everything has an explanation: with Gyaradosite, a base Gyarados can set up while being protected by Wonder Guard or Magic Bounce, then mega evolve to become a Mold Breaker sweeper. Alternatively, Gyaradosite can function with a utility set, which only mega evolves once the user has made sure that the opponent does not have any non-Mold Breaker OHKO moves. This is one of the only two mega stones that I can “recommend” for serious play.
S-: Somewhat established, but almost never used

Sablenite (Magic Bounce)
Sablenite is far less established than Gyaradosite, but does have a niche in being a hard answer to Shadow Tag-based CancerPass teams, and is the only stone on this entire VR that I would consider “underrated”. You don’t lose that much by being base Sableye, since even its mega form heavily relies upon Wonder Guard to not die in one hit, although the extra bulk may come in handy on occasion. If you are running a Sableye-Mega on your team, and it isn’t the level two set, I would personally consider running base Sableye with Sablenite instead, just so you have the extra insurance versus CancerPass.
A: Less established, but are “known” and can possibly work, but would be extremely rare in remotely serious play.

Banettite (Prankster)
Banettite is really held back by Prankster not activating on the turn you mega evolve, but it still has a niche in being a Wonder Guard that sometimes mega evolves to disrupt the opponent with a Prankster Encore, Destiny Bond, and so forth. You need to be VERY clever with how you use this in-game, however, since you won’t be able to use Prankster until the turn after you mega evolve.

Gengarite (Shadow Tag)
Gengar can function as a Wonder Guard until it can trap something crucial with Shadow Tag, and abuse the trapped mon with something like Encore, SmashPass, or just go for a Perish Song. I’m not entirely sure how a Shadow Tag that needs to be activated would be useful, but (if I remember correctly) it used to be a recognized set on an older VR, so I’m placing it in A tier.

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Mewtwonite Y (Insomnia)
I have only ever used this on aerobee’s Dual Mewtwo-Mega-X team in order to block Trick, but I never ended up letting my Mewtwo-Mega-X mega evolve. That being said, you do get Insomnia, and while it is a niche ability, being able to only have it when you want it could be very handy in the right situation.

Pidgeotite (No Guard)
If I remember correctly, Pidgeotite is ran in order to turn a Wonder Guard Pidgeot into a “surprise” No Guard. However, any reasonable player is going to see that you are using this trick on team preview, so its surprise factor is limited.

Red Orb (Desolate Land)
Why you would rather use base Groudon with Red Orb is beyond me, especially since the Surge abilities don’t exist, but maybe someone found a niche for this in the past. That being said, it is still more “established” than anything below it, so I had to put it in A.
Everything below this point is purely theoretical and has no known use cases.
B: The most likely of the hypothetical to be able to actually do something, but still mostly hypothetical. Niches are either less useful, have restrictions due to the limited usefulness of the base form, and/or both.

Absolite (Magic Bounce)
I tried cooking with Absolite once, as a Huge Power that could mega evolve if facing CancerPass. I never finished the team, so Absolite remains almost entirely hypothetical for now. It probably is one of the better stones in this tier, but since nobody has ever used it (to my knowledge), it goes in B.

Alakazite (Trace)
This is (to even my own surprise) probably the most-likely stone in this tier to be usable. Trace is interesting in this Wonder Guard-heavy format, but it still has obvious issues, such as not being able to very easily control what ability you have. That being said, a good enough player might be able to make something like Prankster or Mold Breaker Alakazam @ Alakazite work, since your opponent will likely always switch their Wonder Guards out when it is in (due to them not wanting you to have an extremely fast Wonder Guard that they potentially cannot hit without risking a Mold Breaker). This means that one could potentially use Alakazite to force certain switches from your opponent, and take advantage of them by mega evolving at the right moment to score a surprise OHKO (after all, base 175 SpA and base 150 Speed is rather decent). If someone ever uses this, please let me know, since I would be very curious to see how it does in practice.

Blue Orb (Primordial Sea)
This has the same issue as Groudon @ Red Orb, except now you don’t even get the free immunity to a type that otherwise does quadruple damage versus you. However, this is probably better than anything in C, so I’m leaving it in B. Please do not use this.

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Charizardite Y (Drought)
One of my ideas for for Charizardite Y was to run it on a Sun team as a Wonder Guard base Charizard that can serve as a backup Sun setter, which can be activated once you are sure that there aren’t any OHKO moves and that you no longer need the second Wonder Guard. However, Sun teams are never seen in serious play, so a Drought setter’s usability is very limited to begin with. Maybe one could run it as the sole Drought setter, and have Charizard serve as a “standard” Wonder Guard the rest of the time?

Diancite (Magic Bounce)
I have only ever considered this set once, and it was on a team that desperately needed the extreme role compression that Diancite could provide, since it would allow Diancie to serve as Trick-blocker that usually is Wonder Guard, but can switch to Magic Bounce versus non-No Guard or CancerPass teams (or whenever Magic Bounce is really needed over a second Wonder Guard). That team never saw the light of day, so I never got to try out Diancite, but who knows? Maybe one will need the extreme role compression that Diancite can provide.

Gardevoirite (Pixilate)
My idea for Gardevoirite was to allow a Wonder Guard base Gardevoir to unleash a surprise Pixilate Extreme Speed or Boomburst. If Gardevoir-Mega was faster than Rayquaza-Mega, I would probably try it out, but since it is (sadly) base 15 Speed slower, I don’t really see the point in it.

Manectite (Intimidate)
Base Manectric is still decently fast and can still do some okay damage, but now it can mega evolve to drop Attack in a last-ditch effort to not lose the game. However, Substitute blocks Intimidate, so that further restricts Manectite’s already-limited usefulness.
C: Purely hypothetical, with very specific uses. Do not use one of these unless you desperately need the exact niches that BOTH the mega and non-mega forms have. If anything in this tier wins a game in ORAS PH Occasional I, and it is crucial to winning the game, I will post a recording of me trying to do a backflip in the Pure Hackmons Discord server.

Aerodactylite (Tough Claws)
Aerodactyl could maybe serve as a Shadow Tag that mega evolves to deal a little extra damage in order to get an OHKO? Why you would need that specifically, I have no idea.

Altarianite (Pixilate)
It has Pixilate. That’s literally it. I have no ideas for this thing.

Galladite (Inner Focus)
Inner Focus would let you escape a Fake Out-using Shadow Tag Slaking / Regigigas. However, while that trick may be neat, it means you have to run a base Gallade on your team, which definitely is not worth it.

Glalitite (Refrigerate)
It has Refrigerate. It can probably also be something other than dead weight as a Wonder Guard in its base form.

Heracronite (Skill Link)
Water Shuriken and Icicle Spear become five hits, and can be a possibly not-useless Wonder Guard. Why is this useful, you may ask? I do not know.

Kangaskhanite (Parental Bond)
With Fake Out, your first hit can (theoretically) break a low-HP Substitute, and the next hit would cause the opponent to flinch.

Mawilite (Huge Power)
It has Huge Power, and could possibly be a passable Wonder Guard pre-mega evolution.

Medichamite (Pure Power)
My idea for Medichamite is the exact same as my idea for Mawilite, except for the user being Fighting / Psychic-type instead of Steel / Fairy-type.

Pinsirite (Aerilate)
It has Aerilate, and can probably be a not-unusable Wonder Guard in its base form. If Excaliju can win with Wonder Guard Machamp, I don’t think Wonder Guard Pinsir is that out of the question of what can be used.

Salamencite (Aerilate)
It also has Aerilate. This could possibly work alongside Gale Wings and Boomburst (allowing you to have a priority Aerilate Boomburst), but I’m pretty sure this doesn’t actually work. It would be reallyfunny if it did, however.