A long - lost relative of Sirius Black could be a shapeshifter. Since the Blacks had many secrets and were involved in a lot of dark magic in their family history, it would be believable for a hidden relative to have this unique ability.
A character who was experimented on by a dark wizard in their past. The dark magic used in the experiment could have given them the shapeshifting ability. This character would then have to deal with not only the physical changes but also the emotional and moral implications of having an ability that was forced upon them. They might struggle with whether to use it for good or be tempted by the power it gives them.
Maybe there could be a new antagonist, a dark shapeshifter who is jealous of Harry's power. This antagonist could be an old wizard who has been in hiding for centuries, waiting for the right time to emerge and take over the wizarding world. He sees Harry as a threat to his plans and tries to eliminate him.
Perhaps Voldemort himself, even though he doesn't have the natural ability to shapeshift in the original story. In the fanfiction, he could find a way to mimic shapeshifting, or he could use dark magic to control other shapeshifters to go against Harry. This would make for an even more dangerous and complex conflict as Harry has to face his greatest enemy with this new aspect of his powers.
Perhaps Harry discovers he's a shapeshifter early on in his Hogwarts years. He might use this power to play pranks on the Slytherins by changing into a mouse and scaring them in the dungeons. But then, he realizes he has a greater responsibility when he sees the potential of using it against the Dark Arts. For example, he could shift into a large and powerful animal like a hippogriff to fight off Dementors.
Obviously, Ash's main friends like Misty and Brock would be involved. Misty could be the one who first notices Ash's strange new ability and helps him keep it a secret at first. Brock, with his knowledge of Pokemon, could try to figure out how to reverse the shapeshifting if it becomes a problem. Also, Team Rocket would be great characters in this. They could constantly be trying to capture Ash in his different shapes, thinking they've discovered a new and valuable Pokemon.
She could take the form of a cat. Cats are known for their stealth and curiosity in the wizarding world, which would be useful for spying. Just like how Crookshanks was able to sense things others couldn't, Fem Harry Potter as a cat could prowl around Hogwarts or other places undetected.
Well, I think Orion is a really cool shapeshifter character in some fanfics. He can shapeshift into a powerful dragon, which is just amazing. He uses this form to protect Hogwarts from various threats. There's also a character named Nova. She's a shapeshifter who has the ability to blend into her surroundings by taking on the appearance of the environment. It's a very unique power that she uses in really clever ways. And let's not forget about Thalia, who can shapeshift into different mythical creatures and often uses her powers to help her friends in their adventures.
One interesting concept could be a shapeshifter who uses their ability to infiltrate the Death Eaters. They could change their appearance to gather intelligence and help the Order of the Phoenix.
In 'Harry Potter Shapeshifter OC Fanfiction', a frequently seen theme is the exploration of the 'other'. The OC, being a shapeshifter, is different from the regular wizards and witches. This difference often leads to themes of isolation at first. They feel alone because they are not like the others. Another theme is the journey of self - discovery. Through their shapeshifting experiences, they learn more about themselves, their limits, and their potential. And of course, the theme of magic itself. How their shapeshifting ability interacts with the existing magic in the Harry Potter universe, whether it complements it or clashes with it, is also a common exploration in these fanfictions.
The Children of the Forest are also common in shapeshifter fanfictions. They are already somewhat mysterious in the original series, and their connection to magic makes them ideal candidates for shapeshifting in fan works. Writers might have them shapeshift into different animals or even blend in with humans to protect their secrets or fight against their enemies.
Perhaps a group of rogue ninjas who are threatened by Naruto's shapeshifting ability. They might see him as a threat to their plans and try to eliminate him. They could be led by a mastermind who is well - versed in countering shapeshifting techniques.