Okay... instead of starting an argument that although would be reasonable, I can guarantee will just end in a flame war... I'm instead going to write out some points in the pokemon franchise that prove that Pokemon and Human Fusion is a possible future feature that could happen and wouldn't be out of nowhere as well as it
definitely would not ruin the Pokemon Series.
First off, lets talk about the wondrous lore of Sinnoh.
The Myths of Sinnoh tell many fascinating lores about the origins of people and pokemon and how their world began. I would like to focus first on the second and third Sinnoh Folk Tales.
These two folk stories of Sinnoh depict an interesting concept that in the ancient times of Pokemon and Humans, they were one and alike and could not be distinguished from one another. This means that the Humans of the Pokemon World are humanoids that evolved from ancient pokemon and then eventually genetically segregated themselves as a unique species apart from pokemon.
Why does this matter? Well if the Humanoids of the pokemon world are genetic ancestors to pokemon and are more or less in their own special way a species of Pokemon, then logically you could assume that they could fuse with a pokemon just as though a pokemon could fuse with another pokemon. Game Freak has already laid the groundwork for the magic pseudo-science with their folklore giving us a good indication that Pokemon and Humans were once the same creatures, so who's to say that they couldn't set up a mechanic within the series that says a human can bond with their pokemon far beyond the normal Mega Evolution and then fusing them together. If pokemon can fuse together, and people are ancient pokemon, then why shouldn't they be able to fuse with Pokemon?
This concept with using the Folk lore of Sinnoh to explain Pokemon Human fusion would definitely be a great way to further expand the mysterious lore behind the origin of the pokemon world as well as the connection between Pokemon and Humans, and honestly, further expanding this idea and using it as a game mechanic I really think would fascinate pokemon game players. One of the most common things I hear about people loving the Pokemon Franchise is that they love feeling like they are bonding and becoming closer to their pokemon in the games, well Pokemon Human Fusion in my opinion would be the ultimate test and proof of a bond between a Trainer and their Pokemon.
My next major point is something that I personally dislike within the Pokemon series, but it still proves that Pokemon and human fusion is something that could be a legitimate game feature.
Ash-Greninja.
Ash's Grenininja has a bond with Ash that although is not an actual physical Fusion between the character and his pokemon, it honestly might as well be due to appearance and the overall lore of how this pokemon functions alongside of its trainer.
As the
lore of Ash-Greninja explains:
"... This form comes with a side-effect: whenever Greninja takes damage, the pain is shared with Ash in the same location where Greninja was hurt. Ash is also shown to be able to see from Greninja's point of view and will occasionally mimic Greninja's movements. Ash describes it as feeling like he "became" Greninja. It is revealed by Clemont's device that Ash and Greninja share exactly the same heartbeat while Greninja is in the form and that the form can be accessed when Ash and Greninja's feelings synchronize. If the form is pushed too hard, it is possible for Ash to pass out from the resulting exhaustion and pain. ..."
This proves that during the Battle Bond of Ash and Greninja, they are basically bound to one another as a single entity temporarily and they share their emotions and physical stress. This, despite not being in a single physical body, is Fusion between a pokemon and human. Debate the semantics of it as much as you please, but all in all the only thing that makes them not fused is their physical bodies, everything else is a fusion of power, emotion, and mentality.
Ash-Greninja is incredibly popular in both the animated series as well as in the actual Pokemon games. Despite the fact that it is a slightly modified form of Mega Evolution, it can easily be re-imagined in a future pokemon game to funtion as an actual Human and Pokemon Fusion that can be supported through the Pokemon Amie or Refresh function which at this point seems like not only a staple in the main series but already has features that heavily sway battles in the players favor. If a bond between a pokemon and a human can swing magic criticals and stop a pokemon from being knocked out, then who's to say that the bond couldn't cause a mysterious fusion between the trainer and pokemon?
Considering that Pokemon Amie also has unique dialogue after maxing your pokemon bond, with commentary such as how you and your pokemon are breathing at the same time and such, it wouldn't be that far of a leap to say that when you are completely in sync with your pokemon you could achieve a similar bond between you and your pokemon that would be similar to the relationship between Ash and his Greninja, the next logical step up from this bond would be actual fusion.
I must state again, I'm not the hugest fan of a pokemon fusion mechanic... however I can easily identify and acknowledge the evidence that this could be a legitimate future feature in a pokemon game, because Game Freak chose to write their games with this lore in it. If they wanted this mechanic, they could do it and say, "Look at the Lore! This mechanic easily fits into our world because we already wrote lore to support this feature!"
It was probably not the intention of this lore to build this mechanic, but adding this mechanic would fit into the lore they have already built, and they wouldn't have to struggle to make it work.