Hawk Moth could still be the main villain. He might target Tom specifically because of his relationship with Marinette (Ladybug). Maybe he thinks that by akumatizing Tom, he can get closer to getting Ladybug and Cat Noir's miraculouses.
Another option could be an old acquaintance of Tom's who has a grudge against him for some past event. This person gets akumatized and their power is to turn everything they touch into dough, which causes a lot of problems in Paris. Ladybug and Cat Noir must stop this villain while also trying to figure out how to reverse the effects of their power.
A new villain could be introduced. For example, a jealous rival baker who wants to put Tom out of business. This villain could team up with Hawk Moth and use akumas to create chaos in Tom's bakery and around Paris, forcing Ladybug and Cat Noir to deal with both the akuma - created problems and the business - ruining schemes.
It could still be Hawkmoth. Since he's always after the Miraculouses, he might find a way to turn Cat Noir evil and use him as a tool to finally get Ladybug's Miraculous. So Hawkmoth would be pulling the strings in the background while Cat Noir does his dirty work.
Definitely Tikki and Plagg. These Kwamis are essential to the Miraculous universe. Their interactions with Cat Noir and Ladybug, as well as their own little quirks and personalities, can make for really fun fanfiction. Additionally, Sabrina, Chloe's sidekick, can be an interesting character to include. She could have her own story of breaking free from Chloe's control or getting involved in the hero - villain drama in some unexpected way.
One idea could be that Tom discovers the secret identities of Ladybug and Cat Noir. He could then get involved in helping them protect Paris in his own unique way, like using his baking skills to create special treats that give them extra energy during battles.
There's a fanfiction called 'The Miraculous Origins'. It delves deeper into the backstory of the Miraculous. It explores how the Miraculous were created and how they came to be in the hands of the current holders. It's a long read but worth it for fans who want to know more about the lore of the Miraculous Ladybug Cat Noir universe.
One possible plot could be that Hawkmoth manages to finally corrupt Cat Noir. He might start causing chaos in Paris, attacking Ladybug instead of helping her. Ladybug would then be in a difficult situation, having to either try to save her partner or defeat him to protect the city.
One consequence could be a power imbalance in Paris. Since Ladybug's Miraculous is crucial for maintaining the balance against evil. If Cat Noir takes it, he might not be able to use it as effectively as Ladybug, leading to potential chaos as Hawkmoth could take advantage.
If Cat Noir steals Ladybug's Miraculous in a fanfiction, it could lead to a dark turn in the story. Maybe he's being controlled by an outside force. This would shatter the trust between them and change the whole dynamic of their fight against evil. The city would be in more danger as Ladybug is a key protector.
A new character could be introduced as the main villain. Maybe a corrupt police officer who turns to the dark side. He could use his knowledge of the law and the city's security systems to commit crimes and frame the Miraculous heroes. He would be a formidable opponent as he has insider knowledge.
An evil alien warlord could be the main villain. This warlord could be bent on conquering Earth and other planets in the universe. He might see the Miraculouses as a threat and try to destroy Ladybug and Chat Noir first.
Hawkmoth is an obvious choice. His whole mission is to obtain the Miraculouses, and in a death fanfiction, he could succeed in a more devious way than in the original series, directly or indirectly causing Ladybug's death.