Perhaps Plagg cries because he feels guilty. He might have made a mistake, like accidentally revealing something to the wrong person about the Miraculous powers or secrets. This guilt could be overwhelming for him. Also, it could be that he's sad about the state of the world in the Miraculous Ladybug universe. There are so many akumas and problems, and he might feel powerless to really change things in a big way.
First, describe his physical appearance when crying. Say something like 'Plagg's big green eyes were filled with tears, his little body shaking as he sobbed.' Then, show his internal thoughts. For example, 'Plagg thought about how alone he felt, even with his chosen wielder, in this big, dangerous world.'
One idea could be that Tikki and Plagg get accidentally switched with two other kwamis for a day. This would lead to chaos as Ladybug and Chat Noir have to deal with the different powers and personalities of the new kwamis. Another plot could be that Tikki and Plagg discover an ancient secret about their origin that could change the way they view their roles as kwamis. Maybe they find out they were created for a purpose other than just powering the Miraculous.
Tikki and Plagg might interact by sharing stories about their past experiences with different holders. For example, Tikki could tell Plagg about a very brave Ladybug from centuries ago, and Plagg could share his tales of mischievous Chat Noirs. They could also argue about food, with Tikki preferring sweet things like cookies and Plagg loving stinky cheese.
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.
Honestly, it's hard to pick the 'best' writers as there are so many talented ones. However, [writer's name 3] has written some really unique fanfictions. They explore the more philosophical aspects of Tikki and Plagg's existence. And then there's [writer's name 4] who is great at writing humorous fanfictions about the two kwamis. Their dialogues are always on point and make the stories really enjoyable.
One popular theme is the relationship between Tikki and Plagg. They are polar opposites, Tikki being the kwami of creation and Plagg of destruction, and exploring how they interact in different situations can be really interesting. Another theme could be the adventures they have with their holders. For example, when their holders face tough villains, how Tikki and Plagg guide and assist them. Also, stories about their origin and the history of the Miraculous they are associated with are quite popular in fanfiction.
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.