Well, for animal transformation fanfic, you could have a story where a person transforms into a cat. They would then have to adapt to the feline lifestyle - climbing trees, chasing mice (or toys that look like mice), and napping in sunny spots. Or perhaps a transformation into a dragon. This would involve dealing with the huge size, breathing fire (if it's that kind of dragon), and being feared or revered by the people in the story's world. It could be really cool to write about how the character copes with this new identity.
One idea could be a character transforming into a phoenix, gaining the ability to fly at great speeds and be reborn from ashes. Another could be turning into a mermaid - like form, enabling them to swim underwater and communicate with sea creatures. And perhaps a transformation into a griffin, combining the strength of a lion and the flight of an eagle.
One idea could be a young protagonist who discovers they have a dormant dragon - transformation gene. They start having strange dreams and then one day, during a moment of extreme danger or high emotion, they transform into a dragon for the first time. Their journey could then be about learning to control this new power and finding out the origin of their dragon - like abilities.
One idea could be a trainer who accidentally gets exposed to a strange energy and starts to transform into a Pokemon. They then have to figure out how to reverse the process while also experiencing the world from a Pokemon's perspective.
Well, one interesting idea could be the concept of an animal remembering its past lives. For example, a cat in the fanfic might recall being a bird in a previous life and having different instincts and experiences that still influence it in its current feline form.
One idea could be a fanfic where the RWBY characters transform into animals. For example, Ruby could turn into a little red fox, with her speed and agility enhanced. Another idea is an animal - centered adventure where the team has to protect a magical forest full of rare and mythical animals. They might encounter a giant griffin or a group of intelligent ravens that can communicate secrets about the Grimm.
One idea could be a character who can transform into solid earth structures, like mountains or cliffs, for both offensive and defensive purposes. This would add a new dynamic to battles in the Naruto universe.
An 'animal body swap fanfic' might include a story about a snake and a mouse swapping bodies. The snake in the mouse's body would be terrified of being so small and vulnerable, while the mouse in the snake's body would have to figure out how to slither and use its new senses. Also, a fish and a turtle could swap. The fish in the turtle's body would be confused by having a shell and being able to move on land, and the turtle in the fish's body would have to deal with swimming in a whole new way and living in a different aquatic environment.
One idea could be a story about a new villager who arrives in the Animal Crossing world and has a series of comical misunderstandings with the lemon - themed characters. For example, they mistake a lemon - shaped house for a real lemon and try to eat it.
Well, you could write a fanfic where the animals find a way to communicate with humans in a more equal way. Maybe they form a sort of alliance for mutual benefit. Or how about a plot where the pigs' leadership is challenged not by other animals on the farm, but by some mysterious force that makes the other animals question their authority more deeply? Also, a story where the animals start to remember their pre - revolution lives more vividly and it causes a split in the animal community could be really interesting.
One idea could be Stuart transforming into a bird. He could use his newfound ability to fly and explore the world from above, encountering other bird species and having adventures in the sky. Another could be him turning into a fish. He'd swim in the pond or aquarium, make friends with other fish, and discover the underwater world's secrets.