Perhaps an evil wizard who has a grudge against Scamp's kind. When Marinette meets Scamp, the wizard starts to target them both. He uses his magic to create all sorts of problems for them, like making the ground around them turn into quicksand or sending magical creatures to attack them. The wizard wants to stop Marinette from helping Scamp for some mysterious reason.
It could be a cruel dog catcher. He's always on the lookout for stray dogs like Scamp. He sees Marinette trying to protect Scamp and tries to outwit her to catch Scamp. He doesn't care about the bond between Marinette and Scamp and is just focused on doing his job, which in this case, is seen as the 'bad' thing.
One possible plot could be that Marinette is out exploring a new part of Paris when she stumbles upon Scamp. Scamp is lost and Marinette decides to help him find his way home. Along the way, they encounter various obstacles like mean dogs or getting chased by a grumpy old man who doesn't like animals in his yard. Marinette uses her creativity and kind heart to overcome these challenges and in the end, they become good friends.
Hawkmoth could be the main antagonist. In the original story, he's always causing trouble. In a 'marinette dies' fanfic, he could finally succeed in his evil plan, maybe by getting his hands on the Ladybug and Cat Miraculouses and using their combined power against Marinette.
A new character could be introduced as the antagonist. For example, a powerful and greedy hyena who wants to take over the Pride Lands. When Mufasa and Simba meet, they have to team up to fight this new threat. The hyena might have some special powers or a large army of his own followers.
It could be a creature born out of chaos. This being might be drawn to the unique energy created by the meeting of Atreus and Calliope. It tries to consume their combined power, causing destruction in their path as they struggle to stop it from achieving its goal.
A Sith Lord who has been manipulating events from the shadows could be the antagonist. This Sith Lord might be trying to use Rey's connection to Anakin to bring about the return of the Sith in a new and more powerful way. He could be using dark side illusions to make Rey and Anakin distrust each other.
Maybe Orochimaru could be the antagonist. He hears about Naruto's encounter with Yagura and wants to use their connection for his own evil experiments. He might try to capture Yagura again (if he was once under Orochimaru's control) or manipulate Naruto into doing something that would harm Yagura and the relationship between the two villages.
She might befriend Iron Man first. Since he's into technology and Marinette is quite creative when it comes to making things. They could bond over creating new gadgets or improving existing ones.
Well, it could be that Marinette accidentally discovers Plagg while looking for something in Adrien's room. Plagg might initially be wary but then realizes Marinette's kind heart. They could team up to play a prank on Chat Noir.
When Plagg meets Marinette, it could be a significant moment for character development. Marinette could learn more about the kwamis from Plagg, which would give her a different perspective on the Miraculous she holds. Plagg might also start to see Marinette as more than just Adrien's friend. He could realize that she has her own strength and determination. This encounter could potentially change the dynamics between them and even affect how they interact with each other in future events related to the Miraculous.
Well, she might show off her amazing design skills and create some unique superhero costumes for the Avengers. Maybe she'd use her Lucky Charm power in a creative way to help the Avengers out of a tough situation, like finding a key component to stop a powerful villain's weapon.