In some fanfictions, Plagg could be crying because of a loss. Maybe he lost a fellow Kwami or something precious to him in an akuma attack. He has deep emotions even if he often appears carefree. His bond with the other Kwamis is strong, and if there's a rift or something bad happens to them, it would be a great source of sorrow for him.
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.
Well, they could get into all sorts of mischief. Maybe they decide to play a prank on their holders. Tikki could use her positive energy to create an illusion of a delicious macaron that Plagg can't resist, but when he tries to eat it, it disappears. And then Plagg could use his Cataclysm to create little harmless 'disasters' like making a small pile of papers look like they've been completely destroyed, just to make Tikki gasp in surprise.
Well, perhaps the fanfiction might be trying to force relationships that don't make sense in the context of the Miraculous Ladybug world. The show has its own set of relationship dynamics and character arcs, and some fanfictions might deviate too much from that. Also, it could be that the fanfiction is too dark or violent compared to the relatively light - hearted tone of the original series.
Perhaps it's a story where Roxas is transported into the Miraculous Ladybug universe and teams up with Marinette and Adrien. He could bring his unique skills from his own world to help them fight against Hawk Moth. There could be a lot of character development as he learns about the power of the Miraculouses and the importance of friendship in this new world. For example, he might initially struggle to understand the concept of the Miraculous transformations but gradually grow to master it with the help of his new friends.
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.
One adventure could be that they accidentally get separated from their holders and have to find their way back. Tikki might use her ability to sense positive emotions to lead the way, while Plagg complains the whole time about being hungry. Along the way, they encounter other kwamis who either help or hinder them.
Well, Plagg might initially be shocked to discover Hawkmoth's Miraculous. He could try to figure out a way to take it away and keep it from being misused. Maybe he'd hide it in a really strange place only he knows about.
In a Miraculous Ladybug sleepover fanfiction, the characters might have a lot of fun. They could play games like truth or dare, which could lead to some interesting revelations. For example, Marinette might accidentally let slip some of her feelings for Adrien.
In the fanfic, it could be related to an emotional turmoil as well. Adrien has a lot of personal issues and secrets. If something really bad happened, like he found out some truth about his family that shocked him to the core, it might have made him distracted during a fight or a dangerous situation. This distraction could have led to him being in the wrong place at the wrong time and getting caught under the rubble when something like an explosion or building collapse occurred.