Every main series pokemon game has its own thematic motif. This is stated in Bulbapedia, I'd say it's really not arguable.
Motifs with links to Bulbapedia summary
Kanto: Genetics and Engineering
Johto: Tradition and Mythology
Hoenn: Nature and Relationships
Sinnoh: History of the Universe, Myths, and legends
Unova: Relationships between Opposites
Kalos: Beauty and Harmony
Alola: Bulbapedia doesn't have anything listed yet and I actually haven't even played Sun and Moon so feel free to enlighten me
Less concretely determined, is the overall theme of any given game. This is what I really want to derive from this thread. I'm not an english major so I can't tell you if I'm really approaching this the right way but my strategy thus far has been trying to link motifs with narratives/character arcs.
As far as Kanto Im not sure exactly what message they're trying to get across. Frankly, I'm amazed they even had the foresight to give their first region a specific thematic motif, as if they planned to make more regions with differing themes from the get-go. Really makes you wonder how much they already had figured out when Ho-oh first soared across the sky in the anime.
However, I believe the narrative journey from the first games provided essential set up for Johto.
In Kanto, your rival goes through journey with little discipline or respect. This leads to the inevitable loss of his championship title. Oak then scolds him for his negligence and explains to him why he would ofcourse lose. Your rival storms off in anger without really learning anything. This parallels Giovanni, upon his defeat he pretends to give up his poor intentions when in reality he hasnt changed a bit.
As we get to Johto it's clear in every detail that time has passed. It's a new region, but connected both physically and personally to Kanto and its past. All the old Kanto folks are still in Kanto and people like Elm, Clair, Silver, Koga, and Lance all have ties between both regions. The motif of this new region has to do with traditon and mythology, though I think the mythology aspect, especially when showcased in a modern setting (like I presume any pokemon game to be in), is mainly there to represent the passing of time.
The introduction of a day/night mechanic, breeding new generations of pokemon, evolution occuring after spending enough time with your pokemon, the Week Siblings, any of the other features the use the clock, Celebi being able to time travel, Hoothoot: the literal clock pokemon, this all has little to do with tradition or mythology.
Time has passed since Kimono girls danced to bring forth mythical beings, and time has passed since the first time you roamed Kanto. Consequently, people have matured as well. Blue still loves battling but has finally gotten over his ego and seems to not take things for granted anymore as he mourns the destruction of Cinnabar Island. Giovanni finally gives up his evil plans, I think. They don't really talk about it. That's not super important though because he's the bad guy, the "what not to be like" guy. Because he had unresolved flaws, he raised Silver poorly, hence Silver being a dick. Despite that, even Silver turned his hatred of the weak into his passion to be strong, ending the potential bad parenting cycle his dad started. (almost)Everyone has matured. Everyone has gotten older. Possibly in real time?
I'm getting tired of writing so I'll skim over Hoenn, don't think there's much more going on here than what is explained by Bulbapedia. It's about exploring all types of landscapes and seeing everyone living happily with nature(Professor Birch especially because he's a rugged outdoorsman and in every game the field of research the prof. studies seems linked to the motif and these people are the highest authority in the land and everyone respects them) EXCEPT for Team Aqua/Magma. For whatever reason they don't want to live in harmony with nature. Rayquaza comes and he's like "dude what are you even getting at here?" and lays the smack down and then they realize water and land form the Reese's Cup that is Earth and Archie becomes Plusle and Maxie becomes Minun, Norman hands them each the Balance Badge, and now everybody gonna have a good time
Sinnoh has a bit more going on. Gamefreak was on a power-creep with their pokemon designs, they had made every animal into a pokemon by now, so they went all out and created God. But I think they went about this a little more gracefully than people give them credit for. As with all the generations, I strongly suggest you read the Bulbapedia blurb about the corresponding motifs. To sum up, symbolism and dialogue throughout the game reference various religions/cultures takes on the creation of the universe and everything in it.
Relating this to the narrative, I'll start with Professor Rowan. Right off the bat you can see he exudes class. Ditching the lab coat he opts for a more gentlemanly outfit than his peers. This look is only complimented by his age, which by the way exceeds that of Oak's. Rowan uses bigger words than Oak, explores more complicated suject matter, and doesn't put up with shit from a 12 year old.
You think he's a formal, conservative, no fun having old man. But im gonna stop ya right there because you're forgetting the important character trait that he also is a big ol fan of Rage Candy Bars.
When you start the game Rowan scolds you and Barry for being reckless. All throughout the story he drills it into your head that you need to act more maturely. This culminates in Barry losing to Team Galactic and realizing he needs to be more realistic. He matures but still desires to be the best. His father "Tower Tycoon Palmer" serves as a look into the future for Barry. They fill out all the same character traits, they have the same desires. Palmer is just older. Barry wants to be like his father, older and more mature but still with a burning passion to battle.
The villain Cyrus on the other hand, once again falls into the "What not to be like" role. When faced with hardship he lost hope rather than reassessing and improving himself. He became a nihilist who failed to see the joy in anything. That's why he failed at destroying the world and that's why Rowan canonically strangled him to death with his big burly arms.
Rowan is a role model. Barry took in his teachings to great avail, as did his previous student Cynthia. Rowan's whole sentiment is that everyone has to grow up at some point, but the excitement doesn't have to end. He's ecstatic every time he comes across a new discovery. There's always something to be explored. Cynthia feels the same and is equally passionate in philosophizing about the out-of-this-world topics that plague the sinnoh region. No doubt, if she had never met Rowan she would have never become champion.
I haven't done much research in to 5th gen or beyond yet and I don't remember them well enough to suggest a theme that occurs in these games. Would love to hear anyone else's insight on these gens or any criticism on my points or really anything that promotes discussion because I don't want this thread to get locked and I'd also really would just love to hear what others think. Though if I were to gather anything from what I've looked into thus far, I'm sure Gamefreak would continue to preach themes of reaching maturity.
A journey in reaching maturity seems cliche, it has ofcourse been done a million times. But with Pokemon specifically I feel Gamefreak does this because of the unique fanbase. In the late 90's everyone got hooked on Pokemon and to this day that crowd still makes up a huge percentage of it's fanbase. I translate this as a comforting message that we all grow up eventually but pokemon will always be there as a means of celebrating youthful joy.
Motifs with links to Bulbapedia summary
Kanto: Genetics and Engineering
Johto: Tradition and Mythology
Hoenn: Nature and Relationships
Sinnoh: History of the Universe, Myths, and legends
Unova: Relationships between Opposites
Kalos: Beauty and Harmony
Alola: Bulbapedia doesn't have anything listed yet and I actually haven't even played Sun and Moon so feel free to enlighten me
Less concretely determined, is the overall theme of any given game. This is what I really want to derive from this thread. I'm not an english major so I can't tell you if I'm really approaching this the right way but my strategy thus far has been trying to link motifs with narratives/character arcs.
As far as Kanto Im not sure exactly what message they're trying to get across. Frankly, I'm amazed they even had the foresight to give their first region a specific thematic motif, as if they planned to make more regions with differing themes from the get-go. Really makes you wonder how much they already had figured out when Ho-oh first soared across the sky in the anime.
However, I believe the narrative journey from the first games provided essential set up for Johto.
In Kanto, your rival goes through journey with little discipline or respect. This leads to the inevitable loss of his championship title. Oak then scolds him for his negligence and explains to him why he would ofcourse lose. Your rival storms off in anger without really learning anything. This parallels Giovanni, upon his defeat he pretends to give up his poor intentions when in reality he hasnt changed a bit.
As we get to Johto it's clear in every detail that time has passed. It's a new region, but connected both physically and personally to Kanto and its past. All the old Kanto folks are still in Kanto and people like Elm, Clair, Silver, Koga, and Lance all have ties between both regions. The motif of this new region has to do with traditon and mythology, though I think the mythology aspect, especially when showcased in a modern setting (like I presume any pokemon game to be in), is mainly there to represent the passing of time.
The introduction of a day/night mechanic, breeding new generations of pokemon, evolution occuring after spending enough time with your pokemon, the Week Siblings, any of the other features the use the clock, Celebi being able to time travel, Hoothoot: the literal clock pokemon, this all has little to do with tradition or mythology.

Time has passed since Kimono girls danced to bring forth mythical beings, and time has passed since the first time you roamed Kanto. Consequently, people have matured as well. Blue still loves battling but has finally gotten over his ego and seems to not take things for granted anymore as he mourns the destruction of Cinnabar Island. Giovanni finally gives up his evil plans, I think. They don't really talk about it. That's not super important though because he's the bad guy, the "what not to be like" guy. Because he had unresolved flaws, he raised Silver poorly, hence Silver being a dick. Despite that, even Silver turned his hatred of the weak into his passion to be strong, ending the potential bad parenting cycle his dad started. (almost)Everyone has matured. Everyone has gotten older. Possibly in real time?
I'm getting tired of writing so I'll skim over Hoenn, don't think there's much more going on here than what is explained by Bulbapedia. It's about exploring all types of landscapes and seeing everyone living happily with nature(Professor Birch especially because he's a rugged outdoorsman and in every game the field of research the prof. studies seems linked to the motif and these people are the highest authority in the land and everyone respects them) EXCEPT for Team Aqua/Magma. For whatever reason they don't want to live in harmony with nature. Rayquaza comes and he's like "dude what are you even getting at here?" and lays the smack down and then they realize water and land form the Reese's Cup that is Earth and Archie becomes Plusle and Maxie becomes Minun, Norman hands them each the Balance Badge, and now everybody gonna have a good time

Sinnoh has a bit more going on. Gamefreak was on a power-creep with their pokemon designs, they had made every animal into a pokemon by now, so they went all out and created God. But I think they went about this a little more gracefully than people give them credit for. As with all the generations, I strongly suggest you read the Bulbapedia blurb about the corresponding motifs. To sum up, symbolism and dialogue throughout the game reference various religions/cultures takes on the creation of the universe and everything in it.
Relating this to the narrative, I'll start with Professor Rowan. Right off the bat you can see he exudes class. Ditching the lab coat he opts for a more gentlemanly outfit than his peers. This look is only complimented by his age, which by the way exceeds that of Oak's. Rowan uses bigger words than Oak, explores more complicated suject matter, and doesn't put up with shit from a 12 year old.
You think he's a formal, conservative, no fun having old man. But im gonna stop ya right there because you're forgetting the important character trait that he also is a big ol fan of Rage Candy Bars.
When you start the game Rowan scolds you and Barry for being reckless. All throughout the story he drills it into your head that you need to act more maturely. This culminates in Barry losing to Team Galactic and realizing he needs to be more realistic. He matures but still desires to be the best. His father "Tower Tycoon Palmer" serves as a look into the future for Barry. They fill out all the same character traits, they have the same desires. Palmer is just older. Barry wants to be like his father, older and more mature but still with a burning passion to battle.
The villain Cyrus on the other hand, once again falls into the "What not to be like" role. When faced with hardship he lost hope rather than reassessing and improving himself. He became a nihilist who failed to see the joy in anything. That's why he failed at destroying the world and that's why Rowan canonically strangled him to death with his big burly arms.
Rowan is a role model. Barry took in his teachings to great avail, as did his previous student Cynthia. Rowan's whole sentiment is that everyone has to grow up at some point, but the excitement doesn't have to end. He's ecstatic every time he comes across a new discovery. There's always something to be explored. Cynthia feels the same and is equally passionate in philosophizing about the out-of-this-world topics that plague the sinnoh region. No doubt, if she had never met Rowan she would have never become champion.

I haven't done much research in to 5th gen or beyond yet and I don't remember them well enough to suggest a theme that occurs in these games. Would love to hear anyone else's insight on these gens or any criticism on my points or really anything that promotes discussion because I don't want this thread to get locked and I'd also really would just love to hear what others think. Though if I were to gather anything from what I've looked into thus far, I'm sure Gamefreak would continue to preach themes of reaching maturity.
A journey in reaching maturity seems cliche, it has ofcourse been done a million times. But with Pokemon specifically I feel Gamefreak does this because of the unique fanbase. In the late 90's everyone got hooked on Pokemon and to this day that crowd still makes up a huge percentage of it's fanbase. I translate this as a comforting message that we all grow up eventually but pokemon will always be there as a means of celebrating youthful joy.