Prior to mega forms, and other such powerful transformations, the only way that creatures in the Pokemon franchise could change is if they were to permanently evolve into their next stage of life. The concept of evolution for all pokemon in video games originated from the metamorphosis witnessed in real-world animals. Like how a caterpillar turns into a cocoon and then into a butterfly. This natural progression of a species, this growth, gives many trainers great satisfaction. Seeing one of their pokemon evolve can evoke strong emotions of pride, relief, and joy at seeing a friend become stronger.
As such, there are many gamers out there who wish that some of their favorite pocket monsters had a bit more to their life cycle to enjoy before reaching their final form. Fire-types are among some of the most used due to their excellent battle potential, therefore a lot of them have won their way into trainers' hearts. Comprising approximately only 8% of the total pokemon population, Fire-type lovers understandably desire evolutions for many that exist as a single-stage or two-stage line.
10 Ninetales
With their elegant appearance and fascinating lore, Ninetales has been a fan favorite since the earliest days of the Pokemon games. They represent a balance between beauty and power from their multiple majestic tails and mighty Fire-type moves like Fire Spin.
They were given some attention in the Generation VII games, Pokemon Sun & Moon, in that they received an interesting new form. Instead of being Fire-type, however, this new form was Ice and Fairy-type. Although literally and figuratively very cool, long-time fans of this fox pokemon are disappointed that there won't likely be any additions to this evolutionary line. A shame, since there is so much potential in some sort of demon or spiritual fox final stage.
9 Arcanine
Another fan favorite from the Generation I games is Arcanine. Initially intended to be a legendary pokemon, as hinted at by their category in various pokedex entries over the years, it's no wonder people gravitate to this awesome canine. They combine elements of many awesome real-world animals like the color and fur pattern of tigers, the mane-like patches of fur found the most on lions, and the body-type of a huge wolf.
In addition to this, Arcanine in both the anime and the games are as strong as they look. With all-around decent stats and access to some unique moves such as Extreme Speed, this furry friend can be a great asset to many teams. Another evolution in this line could result in a therian hulk that could put actual legendary beasts like Entei, Raikou, and Suicune to shame.
8 Rapidash
Flaming steeds that are also based on mythical creatures are pretty hard to top in terms of concept and design. This seems like a decent reason why Rapidash was not given another stage of evolution. Although, as of Generation VIII, fans were happy to see this fire horse gain a new form in the Galar region.
This new form, that of a Psychic and Fairy-type, feels a lot more whimsical and powerful. Fire-type aficionados would much rather see another direct evolution to their original form. Maybe some wings to create a pegasus-like pocket monster or additional legs to resemble Sleipnir, the steed of the Norse god Odin.
7 Magcargo
As opposed to many of their burning brethren, Magcargo specializes in being physically defensive instead of offensive. This has resulted in this lava snail being one of the most overlooked Fire-types in the history of the franchise. Though, if underestimated, Magcargo can scorch foes to ashes with their Lava Plume or Flamethrower.
Sadly, to do so trainers will have to visit previous Pokemon Games, as poor Magcargo is not available in Pokemon Sword & Shield. Though, that does not mean they are gone for good! In the next generation, who is to say that they may not return with another evolutionary stage. One that maybe gives them more defensive stat points and HP in order to truly capitalize on their niche.
6 Volcarona
With an aesthetically pleasing design, excellent battle capabilities, and cool lore that reflects the concepts that resulted in their creation, Volcorona usually tops people's picks for "coolest Bug-type pokemon". In appearance, Volcarona bears resemblance to a very large moth, though also has some mythical characteristics. One of which is that their wings spread out like flares arcing from the sun.
Also, they are arranged in a way unseen in any real moth and bear a closer resemblance to those of a seraph: angels who are often associated with fire. Another evolution for this exceptionally awesome Bug and Fire-type could be a form with even more flare-like wings and yellow on their thorax to be more representative of them being categorized as a "sun pokemon" in a lot of pokedex entries.
5 Salazzle
Without any other pokemon of this type combination to compare them to, Salazzle currently reigns as the best Poison and Fire-type pokemon. Their unique ability, Corrosion, allows this female-only pokemon species to deliver the poison status ailment to other Poison-types and even the normally immune Steel-types. This makes them arguably the best choice for a toxic-oriented strategy in battle.
There is a lot of potential for another evolutionary stage in that there are tons of reptilian beings from mythology to draw inspiration from. Nidhogg, a draconic serpent in Norse mythology, is often depicted gnawing the roots of the world tree Yggdrasil. This massive-scale corruption could lead to some cool dragon-like features being added to Salazzle without actually making them part Dragon-type.
4 Torkoal
Being another defensive Fire-type, like Magcargo, this tortoise pokemon also gets overlooked by everyone who isn't Flannery from Lavaridge Town. Given even less love than the aforementioned lava snail by developers at Game Freak, Torkoal has not received any evolutionary stages or alternate forms.
Taking inspiration from mythology once more, as many existing pokemon do, Torkoal could potentially be given another stage that is reminiscent of Genbu, the black tortoise constellation from Chinese mythology that represents the north as well as winter and is often depicted as a tortoise combined with a serpent. In this new stage, Torkoal could become more covered in black, gain a serpentine neck (not unlike some real tortoise species), and maybe even secondary Ice typing.
3 Heatmor
No one would blame most trainers for needing to look up this pokemon, as Heatmor is very much one of the more forgotten among Fire-types. Due to middling stats in everything, except their decent Attack and Special Attack, this anteater pokemon pales in combat when compared to the more commonly-picked Fire pokemon.
However, they have a versatile movepool that warrants more attention. Their signature move, Fire Lash, is one of the best physical Fire-type attacks because it has a sufficient base power and guarantees that an opponent's physical defense drops. Another stage in this pokemon's line could include more whip-like attachments to their body, as in a dextrous tail or tendrils emerging from the vent-holes on their arms.
2 Turtonator
Many have qualms with Turtonator for being part Dragon-type because there is no clear reason why this should be the case. There is nothing really draconic about them, as they have much more of a resemblance to certain soft-shelled turtles due to their long nasal tube.
However, Game Freak could remedy this quite easily with another evolution. Giving this blast-turtle pocket monster more carnivorous features, such as conical teeth or a sharp beak, could be an easy way to make it appear more draconic. This would be an attempt to emulate the source material for Turtonator's origin better, specifically the Tarasque, a monstrous turtle from French folklore, and the dragon-turtle from Chinese mythology.
1 Heatran
For the most part, legendary pokemon don't evolve. For them to do so too often would mess with the lore set up by the franchise that states these certain species are unique to a single or small group of individuals. However, some of these legendary pokemon are given genders, implying that they might be able to breed.
Heatran, the mighty molten monster of Fire and Steel typing, is one such legendary. They can be found as either male or female, yet cannot actually breed and produce eggs because they are in an undiscovered egg group. This can be used as a portal for Game Freak to introduce sexual dimorphism into this species, which further gives them room to conceptually expand. Having separate female and male evolutions for Heatran can lead to cool next-stages reminiscent of the various volcano gods from around the world, like Hephaestus from Greek mythology and Madame Pele from Hawaiian folklore.