There are many reasons why a Pokemon type might need to be adjusted within the games. Primarily, it's because of their average stats, strengths, and weaknesses. Additionally, some Pokemon are more common than others, which means there should be fewer instances of one type or more of another T. Some fans may be hoping that overpowered types, like Fairy, will become more balanced in Pokemon Scarlet/Pokemon Violet to even the playing field both in-game and competitively.
Rock-type Pokemon may not be as common as Water-type, but many of them have type combinations that give them double weaknesses; that is, there are certain types that deal 4x damage against them. For example, Carkol and Coalossal, introduced in Generation 8, are Rock/Fire-type, giving them a double weakness to Water-type moves. What's more, given their poor Speed, they are less likely to move before their opponent. This low Speed is a common stat among Rock-types, though they do have high Defense or Special Defense. Rarely do Rock-type excel in the Special Attack or (of course) Speed.
Scarlet & Violet could fix this by creating a dual Rock-type Pokemon that doesn't have a 4x weaknesses. Alternatively, they could switch up the formula by offering up a Rock-type that has more Special moves and a solid base Speed.
The Poison-type Pokemon are commonly paired with Bug-types. For the longest time, Poison-type attacks were only strong against Grass-type Pokemon, until the Fairy-type in Generation 6. Unfortunately, they're weak to Ground and Psychic-type Pokemon, the latter of which is quite popular among trainers.
While Poison-type isn't going to be losing its weakness to Psychic any time soon, it wouldn't be a bad idea to create a new dual-type Poison Pokemon, such as Poison/Ice-type. At least with a second Ice-typing, it could have STAB coverage for Poison's Ground-type weakness. Since Fairy-type are becoming more common in battle, more trainers have flocked to adding Poison or Steel-type Pokemon to their Party. Buffing the Poison-type would be a move in the right direction for the Pokemon world.
The mysterious Dark-type is home to fan-favorite Pokemon such as Zorua and Zoroark, and Umbreon. They're a good counter for Psychic-type and Ghost-type Pokemon. However, as time has gone on, Dark-type Pokemon more or less faded back into the shadows. Their new weakness to the Fairy-type Pokemon has not done them any favors.
One way to fix the Dark-type is by cutting back on its common Dual-type Pokemon. There are many Dark-types with Fighting as a primary or secondary Typing. While a Dark/Fighting-type may have an immunity to Psychic-types, it's still weak against Fighting-type moves, and has a 4x weakness to Fariy-types. This can be mitigated by giving Dark-type Pokemon more coverage in their learnset. Even making a new combination such as a Bug/Dark-type Pokemon could neutralize its Fighting weakness while giving it an advantage against other Dark-type Pokemon.
Ice-type Pokemon have a reputation for their strength over Dragon-type Pokemon. However, the Fairy-type seems to have taken this title in more recent entries. Ice-type are the least common Pokemon in the games, making them quite rare in the wild. They do suffer from a weakness to favorite types such as Fire, but they're also strong against common types like Grass and Ground.
The Ice-type can be given the attention it needs simply by making Ice-type more common in the wild. While it may not be fair to put Ice-type in the beginning route, making them available when the level of a trainer's party is in their 20s would be a welcome change to the Pokemon world. Alolan Vulpix and Alolan Ninetales seemed to be popular among the fanbase for their looks and their Ice/Fairy-typing, but they're regional variants. Adding a new Pokemon with a secondary Fairy-type, or other defensive secondary typing, would be beneficial for Ice-type Pokemon.
When trainers are asked what they think is the weakest Pokemon Type, it's usually the Bug-type that comes to mind. The base stats for Bug-type average out around 470, which is lower than other Pokemon Types. In each generation, it's common to see Bug-type in the beginning routes to help trainers get started on their journey. However, they are rarely kept in the party for the duration of the game, and are usually stuffed into the PC before the midway point. They also have several common weaknesses.
Since Psychic and Dark-type Pokemon are weak to Bug-type moves, it would be nice to see a Bug-type Pokemon that could hold its own against them, instead of equipping non-Bug Pokemon with Bug-type moves. There also needs to be less of the overused Bug/Flying dual-type. Against a Rock-type, Pokemon like Butterfree and Scyther are easily squashed in one hit. Base stats also need to be improved for new Bug Pokemon. Make them more durable in exchange for power, or make them stronger in exchange for defense.
Unlike the other types, Fairy-type isn't listed for its poor base stats or its weaknesses. Fairy-types were created to give Dragon-type a new Type weakness, but many argue that they've grown too powerful. They're strong against Fighting, Dark, and Dragon-type with a weakness to Poison and Steel-types, the latter two being fairly uncommon. A Fairy-type with the right dual-typing can be nearly unstoppable in battle.
For example, Crowned Sword Zacian is a Fairy/Steel-type that resists both Fairy type weaknesses. Instead, it becomes weak to Ground and Fire-types. Even more frustrating, Zacian's Speed and Attack are incredible (though this is understandable, considering it is a Legendary Pokemon). The Fairy/Steel dual Typing alone is common competitively (for obvious reasons) and in need of a desperate counter Pokemon. Instead of creating a new type, Scarlet and Violet can instead introduce a Pokemon that's super effective against some of Fairy-type's more frustrating dual Types (i.e: Fairy/Steel, Psychic/Fairy). While most types on this list need a buff, Fairy-types need to be taken down a peg.
Pokemon Scarlet and Pokemon Violet are set to release in late 2022.