Many people in the OM Room chat seem to be doomily convinced that Mix and Mega will and must become a Pet Mod in Gen 8, but I couldn't source this to any announcement, and by itself I found it a highly confusing conclusion to jump to. Even as somebody who doesn't particularly play vanilla MnM, it's quite upsetting to see the state of the room as it anticipates this.
My understanding is that MnM can and should remain an OM with a very small alteration to the premise, along the lines of the following:-
Mix and Mega is an Ubers-based metagame that allows you to use any extant Mega Stone (or Orb) on any fully-evolved Pokémon, with the usual limit of one Mega Evolution removed. When that Pokémon then Mega Evolves or undergoes Primal Reversion, it undergoes the same stat modifications that the original user of that Mega Stone or Orb does did in Gen 7, along with its ability and any type changes that the Mega Evolved or Primal Reversion form has compared to the base species.
Rarely is specific reasoning given as to why MnM could not function as an OM in Gen 8, but here are those that I have seen (or could reasonably imagine), and why I believe they are mistaken:-
1. The Mega Evolution mechanic is absent in Gen 8, therefore a Gen 8 metagame based around it cannot be an OM.
We already have specific precedents to the contrary. Probably the clearest counter-example to start off with would be
Gen 7 Inverse. This metagame is based on the following premise: "Remember that guy in
XY's Battle Maison that you used to pwn, the one who could invert the typechart? What if he ran a competitive meta?" I.e. the core mechanic is specifically taken from a concept that existed in Gen 6 but was completely removed in Gen 7, yet nonetheless it was accepted an as OM in Gen 7 with seemingly no controversy.
While highly relevant, this is only the most mild counter-example to this line of reasoning. Other accepted and uncontroversial OMs have been based on mechanics that were inspired by spin-off Pokemon games but never existed in any form in the main series (e.g.
Linked, with a multi-attacking mechanic inspired by Mystery Dungeon) or have completely original battle mechanic concepts as in
Benjamin Butterfree. So this objection seems to be totally arbitrary.
What defines an OM is
not whether it is based around a mechanic present in the vanilla OU of its generation, but whether the mechanic introduced makes "
broad changes off a consistent rule (Or small set of rules)" rather than requiring arbitrary, case-by-case behavioural definitions. But this would not need to be the case for Gen 8 MnM, where the same effects that were accepted in previous generations can be implicitly generalised to apply to any new Pokemon due to the battle fundamentals that they operate on top of remaining the same.
2. Mega Stones specifically are removed from the game, therefore even if we allow the Mega mechanic to return it would not be practically accessible.
Mega Stones are apparently confirmed to be
unreleased rather than non-existent and could be legalised for the OM. There are multiple examples of OMs that have enabled unreleased items: Balanced Hackmons enables them as long as they follow its core mechanic of being hackable into an item slot, whereas Gen 8 MnM could systemically define its allowed subset as those that enabled Mega Evolution or Primal Reversion in Gen 7.
Enchanted Items enabled a more arbitrary set of items with much more arbitrary effects while being accepted as an OM. Another example, by analogy, is how until about one month ago STABmons allowed unreleased moves to be set through STABmons legality, as they fell under the purview of its core mechanic in spite of normally being illegal under any circumstance.
3. Even if Mega Stones could be legalized, they have no functional effect in Gen 8, therefore they are arbitrary as causes of forme changes.
Mega Stones and Primal Orbs in Mix and Mega never had the same effect as they did in standard metagames. While in standard play, Mega Stones cause an evolving Pokemon forme to be substituted for another, in MnM deltas are applied that dynamically alter the pre-existing forme; even when a Pokemon Mega evolves using its native stone, the stone isn't actually doing the same thing that it does in standard. The mechanics of MnM are fundamentally something original that works systemically by analogy to a mechanic in Gen 6 or 7, but in that sense it can be applied systemically in the exact same way in Gen 8 (see 1.) Even the mega evolution limit, which is not considered a possible 'on-cart' change for the purpose of Hackmons metas, is different.
I think there is one slight relevant aspect to this argument, which is that the special case in which a Pokemon Mega Evolving with its native stone has its sprite replaced with the Mega Forme could be argued as arbitrary and incorrect for an OM, but this is not as such because those formes no longer exist in Gen 8, but because isn't a generally-applied quality of the Mega Stone delta. The ability change, however, is a general quality of the delta, and should remain effective under the general concept of the metagame, regardless of whether that ability is normally extant independent of the Stone in Gen 8.
How could Mix and Mega become a Pet Mod, then?
It could achieve this by making arbitrary, non-systemic changes. Here are some examples: making certain Mega Stones only work on arbitrarily defined subgroups of Pokemon (e.g. Scrafty and Sobble can Mega Evolve using Ampharosite but Wooloo cannot; Sobble cannot Mega Evolve with Lucarionite but Yamper can); inventing new Mega Stones that do not reference a coherent set of existing game data (Galarian Rapidashite adds Fairy typing and gives the Pastel Veil ability).
Why does this matter?
Even with the advent of a 'Pet Mod of the Season', forcing MnM to become a Pet Mod would seemingly strip it of its (by popularity) deserved perma-ladder status, while also likely taking the Seasonal spotlight away from Pet Mods like Sylvemons that are genuinely based around ad hoc decision-making. That is not in itself an argument about whether MnM should be a Pet Mod (I believe the above should be sufficient for that); rather, that such a change would not merely be mistaken, but would have such an impact that it would also be unjust.
