Pokemon Scarlet and Pokemon Violet granted evolutions to Pokemon that desperately needed it, such as Dunsparce and Girafarig. Even some Pokemon that had evolutions gained another addition to the family line; namely, Mankey, Primeape, and the new Annihilape. With new evolution methods being introduced the past few generations, there's no shortage of interesting ways to evolve Pokemon from earlier generations.
Arguably, there are some Pokemon who are created purposefully to be more difficult to use in battle. The chances of them getting an evolution are much slimmer than others. These are just a few Pokemon that are still waiting on a base stat boost or an evolution after Gen 9's Scarlet & Violet.
5 Skarmory
At first glance, Skarmory looks like it could tear apart its enemies, but this Steel/Flying-type is overshadowed by Gen 8's Flying/Steel-type, Corviknight. This doesn't mean Skarmory is entirely powerless, as its base Defense is an impressive 140. With access to moves like Spikes and Iron Defense, it makes a good stalling Pokemon in battle. However, its other stats are average. Despite its Pokedex entry claiming that Skarmory can fly 180-190mph, its base Speed is 70, which is only 3 points higher than Corviknight's Speed.
Skarmory doesn't need an evolution as badly as other Pokemon, such as Dunsparce did pre-Scarlet/Violet. However, it wouldn't hurt to have one, especially with there being more Flying and Steel-type moves. An evolved Skarmory could be faster than regular Skarmory at the cost of some of its Special Defense. Or maybe it could be like Chansey and Blissey, where Skarmory outclasses its evolution thanks to a Held Item like the Eviolite.
If Skarmory doesn't evolve, the games could introduce a pre-evolved form instead. Other dex entries from Gen 2 mention Skarmory chicks and how their wings grow harder from brambles. It would be interesting to see a baby Skarmory and how a player might trigger its evolution.
4 Tropius
This fruit-loving Pokemon did make a return in Scarlet/Violet, but without an evolution. Tropius has the Grass/Flying-typing, making it extremely weak to Ice-type moves. Its slow Speed, meager Attack and Special Attack, plus a shallow learnset puts Tropius at the bottom of the ladder. For a Pokemon that has many common weaknesses, Tropius needs a way to counter other typings.
It's difficult to imagine what an evolved Tropius would look like. Right now, Tropius has the appearance of a fully evolved Pokemon even if it isn't. With each passing generation, Tropius received no base stat boosts and was even skipped over on a Mega Evolution, something two other non-evolving Pokemon, Mawile and Sableye, received from Omega Ruby/Alpha Sapphire. The hypothetical evolved form of Tropius doesn't need to have great Attack or Special Attack, but getting its base stats in Defense and Special Defense up to the 100s could work for a Pokemon with so many weaknesses.
3 Sudowoodo
In Generation 4, Sudowoodo got a pre-evolved form: Bonsly. Sudowoodo has poor Special Attack, Special Defense, and even worse Speed, but it has high Attack and Defense. Despite its Rock-typing, Sudowoodo's learnset is mixed, having mostly Normal-type moves and some Fighting-type. The Rock-type moves it does learn all have below 100% accuracy, which isn't good if players are intending to get STAB from Sudowoodo's moves.
The Sudowoodo family could benefit from a third evolution. This could buff its already good Attack and Defense, while giving a boost to its Special Defense. Furthermore, an evolution means Sudowoodo could make use out of the Eviolite item, which increases its holder's Defense and Special Defense by 50%. A Sudowoodo with a working Eviolite would be very tanky, making it a pain for opponents to whittle down its HP.
2 Durant & Heatmor
Neither of these Pokemon appeared in Scarlet/Violet, but they were catchable in Sword/Shield. Despite their appearance, Durant has surprisingly good stats: It has high Attack, Defense, and good Speey to boot. However, it has poor Special Defense, Special Attack, and HP. This means that getting hit with a Special move like Flamethrower (which deals 4x damage to Bug/Steel-type Durant), is a definite KO.
Heatmor is the opposite of Durant, with better HP, very high Special Attack, and good Attack. Its Special Defense is better than Durant's, but its Defense and Speed are not. Fortunately for Heatmor, it doesn't suffer from a 4x weakness.
One idea for Durant and Heatmor is to have its evolutions based on gender. A female Durant could evolve into an ant queen, while a male Heatmor could evolve into a larger, quadrupedal ant eater. However, given Pokemon's history with gender-specific evolutions (such as Combee and Salandit), the gender ratios for Durant and Heatmor would probably be 1:7 (Durant) or 7:1 (Heatmor). Regardless, evolutions for these two Pokemon could help round out Heatmor's average stats and provide some more Special Defense for Durant.
1 Sableye & Mawile
While Sableye returned for Generation 9's Scarlet/Violet, Mawile did not. These two Pokemon were exclusive to Sapphire and Ruby respectively back in Gen 3, and then in Alpha Sapphire/Omega Ruby in Gen 6. They didn't get evolutions. Instead, they got Mega Evolutions, and it's unknown when or if Mega Evolving will be brought back to the mainline Pokemon games. Mawile is a good Physical Attacker with great Defense and Special Defense...but only when it's Mega Evolved. Sableye, when Mega Evolved, gets better Defense, Special Defense, Attack, and Special Attack, but it has low HP and actually gets slower as a Mega Sableye.
With their average stats as regular Mawile and Sableye, these two need an evolution that can make them stronger without having to rely on Key Stones and Mega Stones. Since its introduction, Mawile has gained the Fairy-typing, making it strong against Fighting-type Pokemon, one of its initial weaknesses. There are plenty of new and improved Dark-type and Ghost-type moves for Sableye, too. Since Mawile and Sableye are seen as "opposites," it's possible these two might get an evolution where they need to be traded while holding an item. Perhaps Mawile needs to hold a Metal Coat while Sableye needs a Reaper Cloth. Unless Mega Evolving returns (which is widely considered unlikely), this Hoenn duo needs a boost to keep it from the lower tiers.