The description of the transformation process is key. It should be vivid and detailed. Instead of just saying she transforms, it could describe the way the magic swirls around her, the feeling of the suit forming on her body, and the power surging through her. Good fanfictions make you feel like you're right there with the character during the transformation.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). There are often a plethora of Miraculous Ladybug transformation fanfictions there. Just search for 'Miraculous Ladybug transformation' in the search bar, and you'll get a long list to choose from.
Maybe a new character from another country comes to Paris. This character has a Miraculous that has powers related to nature. Their role would be to bring a new element to the team. They could use their powers to heal the damaged parts of Paris that were affected by Hawkmoth's attacks, and also help in battles by using nature - based defenses like thick vines to trap villains.
In an akumatized Ladybug fanfiction, the akuma might enhance her powers in unexpected ways. For example, her Lucky Charm could become a more powerful and unpredictable object. Her akumatized form might also have a darker and more menacing look, perhaps with a more aggressive fighting style. Writers could explore how the heroes and villains interact with this changed Ladybug.
Well, I've read a fanfiction where Ladybug got the power of elemental control. She could use fire, water, earth, and air to her advantage. For example, she could use water to put out fires caused by akumas or use air to create a shield. It made the battles more exciting and unpredictable. And in another one, she had the power of telepathy which helped her understand the akumas' motives better and communicate with her allies more effectively.
There could be a fashion designer from a rival school who is jealous of Marinette's swimwear designs. This character could be a bit of a troublemaker and might unknowingly help Hawkmoth in his plans to steal Marinette's designs. But in the end, she could see the error of her ways and become a friend.
A new villain could be a former Miraculous holder who turned evil due to some misfortune. Maybe they felt betrayed by the Order of the Miraculous and now seek revenge. Another possibility is an alien entity that is attracted to the power of the Miraculouses and wants to use them to conquer Earth.
One way could be through the alien invasion itself. Maybe the aliens bring with them some sort of energy that interacts with the existing Miraculous. For example, the Ladybug and Cat Miraculous could gain new abilities like the power to create force fields to protect against alien attacks. Another option is that the Guardians discover ancient Miraculous that were hidden away for a reason and these new Miraculous are specifically designed to fight off extraterrestrial threats.