Mario could be one. Just imagine the chaos if Bowser Jr. swapped bodies with Mario. Mario's friends would be so confused, and Bowser Jr. in Mario's body could cause all sorts of trouble in the Mushroom Kingdom. Another character could be Luigi. Luigi's more timid nature compared to Mario would create a different dynamic if Bowser Jr. swapped with him. And Princess Peach is also a possibility. Bowser Jr. in Peach's body would be quite a sight, and it would turn the typical rescue scenarios on their heads.
A great concept in bowser jr body swap fanfic is the adaptation to new environments. Say he swaps with a character who lives in a very different place like an underwater kingdom. Bowser Jr. would have to learn how to survive and function in this new environment. He might struggle with things like breathing underwater or dealing with the social hierarchies of this new place. This could lead to him developing new skills and a deeper understanding of others, all while trying to figure out how to get back to his original body.
His father Bowser could be a major character too. He might be searching for his son throughout the fanfic. There could also be other Koopa characters like the Koopalings. They could be on a mission to bring Bowser Jr. back or they might be secretly supporting his decision to run away.
One could be his father, Bowser. Bowser might be a main character as he could either be the cause of Bowser Jr.'s crying, like if he scolded him too harshly. Or he could be the one to comfort his son. Another character could be Luigi. Imagine Luigi somehow stumbles upon Bowser Jr. crying and has to decide whether to take advantage of the situation or be kind.
Sometimes, there could be original characters created by the fanfic writer. These could be other magical creatures or even some sort of mentor figure for Bowser Jr. who helps him develop his skills and understanding of the world around him, adding a new dimension to the relationship between Bowser and Bowser Jr. in the story.
Team Rocket members like Jessie and James are frequently included. Swapping bodies could turn their usual villainous schemes upside down. For example, if Jessie swapped bodies with a cute Pokémon, it would be a humorous change from her usual self. And trainers from different regions, such as Misty or Brock, might also be involved as they have their own distinct personalities and relationships within the Pokémon world.
One of the most common characters in spiderman body swap fanfic is Doctor Octopus. His scientific mind and villainous nature make for an interesting contrast when swapped with Spiderman. Also, Venom is a popular choice. The symbiotic relationship and the dark nature of Venom compared to Spiderman's more heroic self can lead to some intense and action - packed storylines when they swap bodies. And don't forget Aunt May, although it might be a more comical or heartwarming body swap scenario compared to the villains.
The Koopalings are also main characters. Each of them has their own distinct personality and abilities. For example, Ludwig is known for his intelligence and musical talent in some versions, and they all play important roles in whatever schemes they cook up.
The main characters are Bowser Jr. and Ludwig. Bowser Jr. is often portrayed as a mischievous and energetic character, while Ludwig is known for his intelligence and musical talents in the Mario universe.
Ben Tennyson would definitely be one of the main characters as he is the central figure in the Ben 10 series. Gwen, his cousin, could also be involved as she is often by his side in the original series. Kevin Levin might also be part of it, especially if the story involves a more complex web of body swaps and relationships among the main characters.
Well, it could involve Wolverine and Mystique. Mystique is a very sexy and shape - shifting character. If they swapped bodies, it would be really interesting as Wolverine would have to deal with Mystique's ability to change forms and her complex relationships. Also, Mystique in Wolverine's body could use his claws and healing factor in unexpected ways.