So, I'm not entirely sure where to put this, but this thread has evolved into a sort of "anything goes general discussion thread" of the subforum over the years. I guess that's just a way to say that we like to nitpick on everything about this franchise, to the point where "this surely must be unpopular, but I think that ..." is one of the most common sentiments a Pokémon fan can have.
So, anyway, the secondary types of fully evolved starter Pokémon. Recently, it's something I've given almost as much consideration as three-stage evolutions or the unfair treatment of Ice-types. I think it's fair to say that we're seeing a bit of repetition in this respect. After Gen IV, which is 18 years ago this year, we've really just seen five of the 15 possible types added to starter Pokémon as secondary typings (OK, if you want to be overly technical, it's 8 out of 18, because the game treats mono-type Pokémon as dual-type Pokémon where the primary and secondary types are the same). But recently, it's been a cavalcade of repetition between Fighting, Dark, Ghost, Psychic, and Fairy, with the occasional starter without a secondary type at all.
One explanation is that Game Freak consider these types "extra cool" and want to use them to give the starters a little bit of extra marketability. But I think a secondary factor also plays in: they want to preserve a rock-paper-scissors relationship between the fully evolved starters. In addition to the usual Fire-Water-Grass triangle, these five types create many possible combinations for other triangles. Fighting-Dark-Ghost is the most obvious one (Hisui, Gen IX), but there have been variants: Fighting-Dark-Psychic (Gen VI), and Ghost-Dark-Fairy (Gen VII). And of course the ever-so-popular nothing-nothing-Fighting of Gen V. Or nothing-nothing-nothing, as was the case in Gen VIII.
The problem is that there are only so many ways to apply five types to three Pokémon, while preserving a triangle, without making a lot of repeats. We've already seen Fire/Ghost used twice in a row, Fighting and Dark have been paired with every type already, Psychic can't fit in a triangle with both Fighting and Ghost or Dark and Ghost, nor can Fairy work with both Dark and Fighting. And if you use Ghost at all, you have to put Dark in there to give it a disadvantage, which locks the third member to Fighting or Fairy so a triangle can be completed.
But wait, why are they sticking to only those five types, again? That, I think is because, not only are those five types extra cool and work easily in triangles, but they have no type chart interactions with Grass, Fire, or Water. They don't interrupt the "sacred balance" of the original rock-paper-scissors typing triangle no matter how you apply them. There's the little exception that Fire resists Fairy, but it's a minor interaction while the other types have none at all (it does mean that whichever 'mon in the secondary triangle is supposed to have an advantage over the secondary type of the Fire starter can't be Fairy, though).
There are some unused combinations that remain, so that Game Freak can continue to use these five types without repeating types too many times. We're only one step away from having used Fire/Ghost as many times as Fire/Fighting. Water/Ghost and Fire/Fairy are still unused combinations for starter Pokémon, although it locks the third type to repeat Meowscarada's Grass/Dark. But what if I told you there was another way?
That's the key part I think I can pass for "Unpopular" enough to be relevant in this thread: the secondary starter type triangle can be balanced in neutrality. Instead of the starters having relative advantages over each other, outside the Fire/Water/Grass triangle, they can be neutral. While preserving Fire/Water/Grass. It's just a bit difficult. Because there's a reason why these five types are Game Freak's go-to types for starters. They don't mess with the FWG triangle, and few other types do that. Either, they give the starters undue resistance/weaknesses to each other's types, or make the triangle unbalanced so that one is clearly "the strong one", or worse, "the weak one".
But I think there are options still. Flying/Normal/Psychic, for instance. The only caveat is that the Grass starter would have to get the Flying type (flying creates an advantage vs. Grass, but neutrality towards the two other). That way, neither starter has relative advantages or disadvantages, and the standard Fire-Water-Grass triangle is preserved. Flying/Normal/Dark works equally well. Electric/Poison/Dark could work too, provided it's Water/Electric and Grass/Poison, so the offensive advantage of the secondary types vs. the primary aren't affecting the balance of the type traingle. Instead of Fire/Dark, Fire/Normal also works in that scenario.
Sadly, there are few types that fit neatly into the Fire-Water-Grass triangle like that. It works if they interact non-neutrally with one of the types, because then it can just be combined with the starter of that type, but far too many types interact with multiple starter types, or with the other possible combinations. Take for instance Ghost and Electric. No interactions with FWG (if the Electric-type is the Water starter), but there's no neutrality to be had with the remaining types (not even the go-to type for neutrality, Normal).
I suspect there are more combinations than I can think up at half past one in the morning on a Sunday, though. Are any of Smogon's many proficient scripters up for the job of discovering neutral starter type triangles, where the only relative advantages and disadvantages between the starters are those given by the primary types of Grass, Fire, and Water?