Here's a long and slightly rambling post about the secondary typings of fully evolved starters. I figured it fits here because it's all about patterns or "rules", which I expect to be broken before long.
So, you may have noticed in recent generations that, despite there being 14 available types that aren't Fire, Water, or Grass, only five of them have been assigned as secondary starter typings since Gen IV: Fighting, Dark, Ghost, Psychic, and Fairy. Of these, Psychic and Fairy have only been seen
once each, while the others have appeared multiple times (Fighting and Dark four times in generations V-IX, counting Hisui, Ghost thrice). What gives? Where are all the other types?
I think it has everything to do with ideas of balance. A quirk of the type chart is that very few types work to set up balanced "triangles" to work with Fire, Water, and Grass. To be specific, I think the designers are working based on four ideas:
- The Fire-Water-Grass triangle must be maintained: Fire must always be super-effective against the Grass starter, and not very effective against the Water starter, and likewise for the other two types.
- The secondary typings should interact with each other, either offensively or defensively, but not create overly lopsided matchups.
- No 4X weaknesses or resistances between the starter types (for instance, Grass/Fighting is 4X weak against Flying, but that's fine as long as there are no Flying-types in the starter triangle).
- Either all starters get secondary typings, or none do.
They have not always managed to keep those rules. In Gen IV, Empoleon takes neutral damage from Grass (breaking rule 1), but Torterra can still hit it super-effectively with Ground STAB. In Gen III, Swampert takes 4X damage from Grass (breaking rule 3). In Gen V, only Emboar has a secondary typing (breaking rule 4). But those examples are quite old. A more recent example of rule 2 being stretched is that Meowscarada and Quaquaval can hit both other starters of their generation with Super-effective STAB, but Skeledirge cannot (although it is immune to Quaquaval's secondary STAB, which I guess balances things out somewhat). And finally, Gen V gave a secondary typing to Emboar but not to the other two. But let's take the rules as general guidelines when exploring the possibilities of starter type diversity.
Right away, we can see that some types don't fit into the starter triangle no matter where we insert them. Dragon resists all three types and would immediately break rule 1. Normal has no favourable offensive or defensive matchups, and so would break rule 2. Rock would create a 4x weakness if paired with Fire or Water, and break rule 1 if paired with Grass. Ground, on the other hand ...
OK, hold on, one thing must be established before we move on to list all the types. Things get a lot easier if we just list the types that have no type chart interactions with Fire, Water, or Grass at all. Aside from the already-discarded Normal, those types are: Fighting, Dark, Ghost, and Psychic. Does the list seem familiar? That ought to put a few pieces together to explain starter typings in recent generations.
There's also Fairy, which is just resisted by Fire, a quite minimal interaction, so it can go into the starter triangle quite easily. Hence Primarina.
However, there are even more intricacies to consider. The five eligible typings of Fighting, Dark, Ghost, Psychic, and Fairy have multiple interactions between each other. If the rule of balance is to be maintained, some of these combinations must be avoided. But herein lies a bit of a challenge: We're running out of good type triangles that don't ruin the relative balance between the three starters. Five eligible types, pick three. Mathematically, there are only 10 possible combinations to be had, and half of them are quite poorly balanced:
- Dark/Fairy/Fighting - Unbalanced. Dark weak against both others, Fairy strong against both others.
- Dark/Fairy/Ghost - Reasonably balanced, used in Gen VII. Dark beats Ghost, the others are neutral.
- Dark/Fairy/Psychic - Slightly unbalanced, not yet used. Dark beats Psychic, Fairy beats Dark, Psychic beats none.
- Dark/Fighting/Ghost - Well-balanced, used in Hisui and Gen IX.
- Dark/Fighting/Psychic - Well-balanced, used in Gen VI.
- Dark/Ghost/Psychic - Unbalanced. Psychic weak against both others, Dark strong against both others.
- Fairy/Fighting/Ghost - Unbalanced. Fighting weak against both others.
- Fairy/Fighting/Psychic - Unbalanced. Fighting weak against both others.
- Fairy/Ghost/Psychic - Slightly unbalanced, not yet used. Ghost beats Psychic, the others are neutral.
- Fighting/Ghost/Psychic - Unbalanced. Fighting weak against both others, Ghost strong against both others.
For those keeping count, that's five that are broken, three that have been used already (one of which twice, in a row), and two that are iffy.
Note that Fairy and Psychic are difficult to balance. Psychic because it has only one favourable offensive matchup, Fairy because it has no unfavourable ones. Hence why they haven't been used as much as the other three.
And of course, limiting the starter trio to Dark/Fighting/Ghost will exhaust the possibilities of unique starter types quite quickly and start racking up multiple repetitions in short order. The only combination that hasn't been used since Gen V is Water/Ghost, which would have then run up against Fire/Fighting (and we're all tired of that) or Fire/Dark (Incineroar again ...), and Grass/Dark (so soon after Meowscarada) or Grass/Fighting (used twice in the past three generations). Likewise, the balanced combinations that involve Fairy and/or Psychic also cause recently used type combinations to be repeated.
Of course, you could just ditch the secondary typings altogether and just rely on the perfect interactions between Fire, Water, and Grass, but that was also done quite recently. And in Gen II. And mostly in Gen V. Besides, it's a bit boring, so I wouldn't consider it a good idea to repeat again.
So in conclusion, I think the TL;DR is:
- Too few types have interactions that work nicely with a Fire, Water, and Grass starter triangle - only five, to be exact.
- Many combination of these types are too unbalanced to work well.
- The combinations that are balanced and work well have already been used.
- Things will become really repetitive if they continue to use them.
I think this post is long enough already, so I'll leave the discussion of the remaining possibilities to a Part 2 post.