Starter Pokemon are a staple of the franchise, and Pokemon Scarlet and Violet's first-partner critters are the latest in the long tradition of giving players a choice between a Grass, Fire, or Water type to start their adventure with. While those three typings are the standard, starters still have the possibility of gaining an additional type as they evolve. In Pokemon Scarlet and Violet, the Grass-type Sprigatito gains Dark in its final evolution, the Fire-type Fuecoco gains Ghost, and the Water-type Quaxly gains Fighting. Even after nine Generations, some types have yet to appear on a starter Pokemon.
The trio of Grass, Fire, and Water has not been the case for every entry in the series. Pokemon Yellow, along with its pseudo-remakes Pokemon Let's Go, Pikachu! and Let's Go, Eevee!, broke this pattern by giving players the Electric-type Pikachu in both Pokemon Yellow and Pokemon Let's Go, Pikachu!, and the Normal-type Eevee in Pokemon Let's Go, Eevee! Discounting those, there are still a handful not yet represented in any starter evolution lines.
Rock-Type
Although commonly used by early-game trainers, the Rock type has never been a part of a starter's evolutionary line. This may be due to the fact that many Rock-type Pokemon like Geodude, Rockruff, and Nacli can be found early in most games, making a starter with the typing unnecessary. The Fire-type starter would also have two 4x weaknesses if paired with Rock, something Game Freak tends to avoid with starters and their evolutions. A Grass/Rock-type starter could be a possibility, and would make a lot of sense for a Pokemon similar to Torterra with it's Ground-typing being easily swapped for Rock.
Bug-Type
Bug is another type common to early-game mons and the trainers who use them. Again, this makes it unlikely for a starter line to gain the type because it's expected that the player will have access to a Bug-type critter early in the adventure. Bug is also one of the worst types in Pokemon due to its weakness against common types like Rock, Fire, and Flying, ultimately giving the Grass starter two 4x weaknesses if paired up. If the Water starter were to gain the Bug type, it would eliminate its weakness to Grass and upset the balance of the starters' type triangle, leaving only the Fire starter with any possibility of pairng well with the Bug type.
Ice-Type
The Ice type is in an interesting situation when it comes to starter Pokemon because it has such a close relation to the Water type. Ice is another infamously poor type choice due to its many weaknesses, but it can be somewhat redeemed when paired with Water as its major weakness to Fire and Steel-type attacks would be eliminated. Ice-type Pokemon are usually found late in the game, meaning a starter that gains the typing could be quite useful for trainers in a variety of situations.
Dragon-Type
Technically, the Dragon type appears on Mega Sceptile and Mega Charizard X, meaning it has been used in a starter evolutionary line. However, given that Mega Evolution is a temporary phenomenon and only present in a limited number of games, this point is debatable. Dragon has historically been one of the most powerful types in Pokemon, which is likely why it has been relegated to only Mega Evolutions of starters thus far. It is unlikely that a future starter would ever get the Dragon typing on a standard evolution because it would eliminate the weaknesses to other starter types and unbalance the triangle.
Pokemon Scarlet and Violet are available now for Nintendo Switch.