I know I've harped on this point a few times before, but I procrastinated today and made a full graphic out of it.
TL;DR, I don't like that new generations continue to use all (or nearly all!) of the old design archetypes, while introducing so few new Pokémon there's barely room to give each type some representation and still do something else afterwards. They should either skip some of the archetypes or introduce more new Pokémon, instead of effectively making nearly the same generation of Pokémon over and over again.
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This is a tangent to your point but I was thinking about this earlier and looking for a place to post it so might as well reply here: I'm starting to think there's another, arguable archetype - the "sub-pseudo" Pokemon. Nearly every generation has that one Pokemon that's "the one to get": not the designated pseudo-legendary, but one that usually takes a lot of time and effort to obtain. It's an arguable category because often more than one Pokemon fits this brief, but thinking about the qualities it possesses, it does seem that there's a pattern:
-it's often elusive (sometimes frustratingly so) and sometimes tricky to evolve
-it's often very desired because of its power, abilities, or popular design
-it's often (but not always) Dragon-type
-it's often the signature Pokemon of the final gym leader or one of the Elite Four
Not convinced yet? Here are the Pokemon I'm thinking of:
Gen 8: either Duraludon or Toxtricity. Duraludon fits the part more on paper as it's Dragon/Steel and has better stats (and is the 8th gym leader's ace) but Toxtricity is hyped up more in-game, being given to you as a gift. Neither is desperately hard to obtain since raids are a thing. I'd go with Duraludon but I could go either way on this.
Gen 7: Turtonator and Drampa both spring to mind, but I'm inclined to say it's actually Golisopod - obtaining a Wimpod is tricky since there's only one area pre-E4 it's found (and it has to be chased first) and its ability makes it tough to raise. Silvally seems the obvious choice but as it's a legendary it's disqualified.
Gen 6: Noivern. Obvious choice; not really much competition here since the Kalos roster is so small. It's relatively rare and very hard to raise due to Noibat being so frail. It's Drasna's ace Pokemon.
Gen 5: Haxorus. Again a fairly obvious choice. I'd imagine a lot of players probably overlook Mistralton Cave altogether, especially on a first playthrough. It evolves quite late (though not as late as most of the Unova lot, it's in the Slow experience group, so takes a lot to raise) and is very powerful when fully evolved. It's Iris and Drayden's ace Pokemon.
Gen 4: Not really one in this gen - Lucario kind of counts but, much like Toxtricity and Silvally, is given as a gift. If I had to think of a Pokemon that fits this criteria it'd actually be Munchlax/Snorlax.
Gen 3: Has two pseudos so it's an interesting case. Flygon is the obvious candidate, but Milotic seems more fitting - it's infamously hard to obtain, just as difficult to evolve, and very strong. Plus it's Wallace's ace.
Gen 2: Kingdra. There's not really much to say about it other than it's extremely hard to obtain and (in Gen 2 at least) is pretty powerful.
Gen 1: Probably the hardest one to judge. At this point, there was only one Dragon-type line so there couldn't be a "lesser" Dragon in the vein of Altaria or Druddigon, which makes it a bit more obscure. People often highlight Arcanine as an example of this since it's classified as a legendary Pokemon, but it is available much earlier than most of the corresponding Pokemon on this list typically are, and if you count Arcanine you pretty much have to accept Ninetales too since they're counterparts. Ultimately I think it's a toss-up between Gyarados and Snorlax, and either could be it, frankly. Gyarados is the original example of the "tediously hard to raise for a strong payoff" Pokemon, while Snorlax is just incredibly strong and rare (but, crucially, not infinitely obtainable). As with Gen 8, I could go either way on this one.
Again, it's not really fitting for this thread, because this is one archetype I really do like, but it's just an observation. I wish they would shake up the archetypes more in future games, and I generally like when they do (Talonflame and Staraptor are particularly good examples of how they've mixed up the formula for the early-game bird Pokemon). What I'd really like to see is more diversity in types in the early-game. This is something USUM does quite well imo; off the top of my head you can get Fire, Water, Grass, Normal, Electric, Flying, Bug, Dark, Psychic, Poison, Rock, Fighting, and Fairy all before the first trial.