Starters: One of the hardest ones to adjust. We're always going to have a Grass, Fire, & Water Starter, let's get that out of the way. You need a Pokemon to start out with, those are 3 basic elements anyone can understand (and how they relate to one another), and most importantly they become mascots of that gen thus VERY marketable. Let's assume their marketable comes via two forms: a cute basic stage & a tough/cool final stage. If they were going to make any changes to the Starters, very low probability but you never know, I would say it's likely them chopping the middle stage. They could do this in two ways: Just make the Starter a 2-stage evolution would be the most simple. OR they could make the Starter a solo Pokemon that has a form change gimmick (Pokemon with multiple forms don't seem to take more than one slot so if you're really tight for room this could be the way to go). Or they could combine the two, a 2-stage evolution with the final stage having a form change, a pseudo way to keep the "3 stages" but without needing to take up a third slot for each (and the third form could also come from the "super mechanic" of that gen like Mega Evo or Gigantamax).
Another even less likely possibility is having a singular Starter but it evolves into different forms. That itself brings in a few number of possibilities, so I'll just cut it here. But you get the picture, they need the Starters and there's a reason for them to be these Types, but they can play around with how many slots they take via going about their progression differently.
Bug & Bird: This one I feel is kind of unfair. Like, I will not deny there is that trope, BUT I'll also say there isn't anything wrong with having it either. There are many kinds of bird and a TON of bugs out there, there is likely going to be bird and bug Pokemon even if they aren't going to be early route. At most maybe they don't need to take up that many slots, and we've had a few instances where they were 2-stage gen birds & bugs, but we're going to have them anyway. And recent gens have done a better job at not only expanding upon their designs but also making them usable whether it's good stats, Typing, or a niche.
For Bugs the "problem" more comes from the actual life cycle of bugs: many have a larva, pupa, and adult stage (not counting Egg for obvious reasons). That's your 3-stage right there. Maybe you can cut out the pupa stage but bugs go through such massive metamorphosis throughout their life it's hard to not base an entire Pokemon off one kind of bug. Because of that we often see the Bug's combined with another concept to give them some visual interest. Blipbug family are ladybugs that are also science fiction tropes, Grubbin family are batteries & railgun, Scatterbug family has the whole different wing patterns depending on real world location gimmick, Sewaddle family has a clothing theme, and the Kricketot family are musical conductors. Now the issue does come with their viability, but that's not technically their fault but rather the box GF put them in. Early fodder Pokemon which evolve fast, that's the gimmick many Bug-types have to deal with and the exceptions aren't early route Pokemon.
Bird have had an interesting time recently, some of them have even become viable options for main game team members & even competitive! While the bugs are combined with another theme, recently GF has been messing with having the bird families starting out as one kind of bird and evolving into a different species of bird. Rookidee family starts out as a chickadee/tit but evolves into a raven/crow (all whom are part of the Passeriformes order), Pikipek family starts as a woodpecker and evolves into a toucan (all members of the Piciformes order), Fletchling family is a red robin that becomes the red-colored peregrine hawk (yeah, no order relation there, I think the relating theme is more of the secondary design of being "flaming arrows"), Pidove family is a pigeon/dove that evolves into a pheasant (once again, no order relation, but rather the link between them is Japanese word puns I kid you not). Heck even Pidgey & Starly go from song birds into birds of prey though more subtlety. Maybe they also can cut it down to 2-stage, though the middle stage now especially serves the purpose of connecting the basic and final stage together since they're different species of birds making the middle a sort of hybrid of both.
But, as I said, they're always going to have a bug and a bird since there's so many design potentials, so it's not bad this group is here especially if they keep up with trying to make them interesting and even competitively good/have a niche.
"Rodent": Unless they have a good design I say the early rodents aren't really needed as much anymore. Like in the early days of Pokemon the early rodents were meant to ease you into the world of Pokemon by being a normal-looking animal with cartoon-y/fantastic features before hitting you with the elemental stuff. But I feel those days of Pokemon are behind us and we've have more interesting and varied Normal-type designs or elemental Pokemon you can easily encounter early-on.
Pikaclone: You know what GF, just stop. While they are a cute group, you pretty much do NOTHING with them. You caught lightning in the bottle once with Pikachu and the closest you've come to that wasn't even with a pikaclone, it was with a Pokemon that WANTED to be a pikaclone (and the pikaclone it was imitating was Pikachu itself). The nail in the coffin for me was the Pikachu Valley in USUM. Like, why just Pikachu? Why not all the pikaclones (and Pichu & Raichu for that matter)? You could still keep the name Pikachu Valley, but just Pikachu felt really gratuitous about Pikachu's mascot title. But if that's your mindset, fine, whatever, but then stop making the pikaclones. You try to shove them in our face but when they no doubt fail you throw them in the forgotten pile. I'll least say the recent pikaclones have been more interesting with their Typing.
"Cutesy" Normal: Like, we're always going to have cutesy Pokemon. They don't have to be Normal-type of course, but on the otherhand that's also saying Normal-types aren't allowed to have cutesy member. I still say this is more of a coincidental group rather then something GF has on a checklist.
Stand-Alone Flying & Normal: Again, I don't think it's something they're consciously doing but it just keeps working out that way, especially for those two Types.
If the Pokemon flies or has wings at least (or is a bird) it'll probably get the Flying-type, that's just how it is. And what better way to make a standalone Pokemon look better than giving it the power of flight. Once again, many species of birds and the gen birds can't have all of them, though at the same time GF probably doesn't want to give them an evolution family if it's just going to go from visually similar small bird to big bird (or has a secondary design theme which wouldn't mesh well with a prevo).
Normal-type is odd because it's considered so many things, even contradictory ones. It's its own kind of Type, it's the "default" Types, it's the gimmick Type, etc.. That last one is most notable as that's usually where the standalone Normal-type comes from, there's a gimmick to it that doesn't match another Type (or its a Type-changing gimmick) so place it in Normal so it can hone in on that gimmick. Also Normal is a hard Type to make a duel-type for as you need a reason for the additional Normal-typing... and even the ones we have there are some you wonder why it has the Normal-typing.
Pseudo-Legendary: Another marketable group, my only problem with this group is they keep making them dragons. Well, I guess that's not the only thing, they have a problem with level progression, when you get them in the game, and keep making the basic stage too weak (we get it, we get it, they're late bloomers, but that doesn't mean the basic stage has to suck). I think the most ideal one they've made was Gible, you can find it half-way through the game (if you know where to look...) and its evolution levels are reasonable. So I think it's a good group to have, teaches the lesson of being patient and helping a weak Pokemon grow will reward you with a Pokemon as strong as a Legendary, but it needs work on diversifying their basis & making getting and training it feel rewarding and not you're hindering your team for a while.
Fossil: I think Fossil Pokemon are alright, maybe needing a shake-up in being more than just 2-stage evolutions (and not what the Galar fossils did...). I think the major problem with them isn't the Pokemon themselves but that every fossil Pokemon needs a fossil item. Maybe it's time we cut back and make two "fossil" items: "Whole Fossil" and "Fossil Half". Whole Fossil you can ressurect a random Fossil Pokemon with one, Fossil Half you need two of them to ressurect a random Fossil Pokemon. To make things interesting, you can only get Aerodactyl with a Whole Fossil while you can only get a Galarian Fossil Pokemon with the Fossil Halves (and, yes, if the Galarian Fossils weren't a thing I would only have "Whole Fossils"). Heck, maybe if they did that they may make a few more Fossil Pokemon you can only get via a Whole Fossil.
Trio Legendary: Should we count the Galarain Birds as Galar's Trio even though they're Regional Variants and got introduced in DLC thus not being part of this "limited Pokemon slot" issue technically? Because if we don't then we haven't had a Legendario Trio for 3 gens now. Gen V sort of overdid it with 2, and the closest we've had were the Island Guardians in SM (but there was heavy emphasis on them which makes them feel like they had a meaningful purpose in the story; most times people have problems with Legendaries because they don't have a role in the story). Honestly I don't think its a group we need to worry about anymore as feels like GF decided to pull back on Legendaries just in general and only including them where they feel they need to have more than just the box legends.
Box Legend & Sequel Legend: Marketable group, at least for the initial Box Legends. Like, I can understand people's boredom with the newest Pokemon games front cover being just the Box Legend you can catch in that game instead of having something more visually interesting like with the Mystery Dungeon and Ranger games. But at the same time, since GF knows the games are going to sell, they know they don't need to make a fancy box cover so instead advertise the Pokemon which are the face of that generation even moreso than the Starters. It's a marketing choice which has worked out for them (if not a bit arrogant feeling). I can also understand people getting tired of the box legends always being involved in the story, and while I do think they can do a story without the box legend, I can still understand GF wanting to adding them so that players have this super powerful Pokemon to look forward too as well can be used to world build. They have draw factor to them, either in getting someone to buy the game or keep on playing through it until finding it.
Mythicals: The last marketable group, pretty much the "forbidden apples" that keeps people playing/keeping their games even after the main story is done, all side objectives are complete, but they have no interest in the competitive scene. Mythicals keep the games in even the most casual video game player's collection as they know down the line there's this strong Pokemon which will have a limited release and who knows when else they'll be able to get another one. Of course, as you may be guess, my problem with Mythicals isn't there existence or use but that there's very little story & lore around them. In addition to getting it I wouldn't mind if they also added in a small story campaign which unlocks or gets downloaded with it that's about either catching it or something you need to do with it once you get it. Because as of late there's been a few Mythicals I don't really get that "Mythical" vibe off of and kind of need to see why these Pokemon have such a high-and-mighty title.