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.
One idea could be to start with Chat Noir being in love with Marinette first. He might be more forward in his pursuit, which could throw Marinette off at first as she's used to being the one with the secret crush. For example, he could leave her little gifts as Chat Noir, which would make her both flattered and a bit confused.
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.
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.
Adrien, in a reverse love square, could become more empathetic towards Marinette's feelings. He might notice things about her that he didn't before, like how she gets nervous around him. This could make him a more understanding friend and potentially a better partner later on. He would also have to deal with his own insecurities about whether Marinette likes him for who he really is or just because he's a famous model.
In this 'Miraculous Ladybug Criminal Ladybug Fanfiction', there could be a lot of potential storylines. One possibility is that an external force is influencing Ladybug to commit crimes. It could be a new villain who has found a way to manipulate the Miraculous. This would create a great conflict as Ladybug has always been the symbol of good. Her friends and allies would be in shock and would have to find a way to stop her without hurting her. The story could also explore her internal struggle, as she might be aware of what she's doing but unable to stop herself.
A new villain could have a weapon that specifically targets Ladybug's powers. When she tries to use her yo - yo or her powers of creation, it backfires and causes her harm. This would force her to rely on her wits and Chat Noir more than ever. They would need to find a way to neutralize the weapon and heal Ladybug.
In a fanfiction, Ladybug could get hurt as a result of a trap set by Hawkmoth. He might have studied her fighting patterns and set up a situation where she is lured into a place full of obstacles that cause her physical harm. This would not only test her physical strength but also her mental resilience as she tries to figure out how to escape and continue her fight against evil.