I'll do my best.I was wondering regarding the line between OMs and Pet Mods, although I think it'd probably end up being the latter. I was spitballing a concept for a metagame where the core change is that the type chart is modified. It's arbitrary; two types are combined, a new type is introduced, and a handful of type matchups are adjusted, ex. Bug and Poison having a mutual weakness like in RBY.
While I think you could potentially handwave that as being singular enough to be considered OM territory, the considerations that would have to be done would be similar in scale to the Fairy bump in Gen 6, and thus be considered a Pet Mod. Existing Pokemon would have their types changed, a handful of moves would get the same treatment, possibly a handful of new moves added, etc.
Ideally you should be able to explain an OM with one sentence, that being the one mechanic change or rule of the om. See for example the currently existing om.
Metagamiate: Every Pokémon gains an intrinsic -ate ability matching its primary type, or its secondary type if shiny.
Stabmons: Pokémon can use any move of their typing, in addition to the moves they can normally learn.
Mix and Mega: Mega Stones and Primal Orbs can be used on almost any fully evolved Pokémon with no Mega Evolution limit.
If you just say "The type chart is modified" to someone they obviously don't have enough information. Your meta's rule is not a consistent rule but rather a set of arbitrary changes. So it's a pet mod.