Maybe a new character could be the main antagonist. A powerful sorcerer from a far - off realm who has heard of the power within Thor and Loki's sister. He wants to harness that power for himself and will stop at nothing to get it. He could create all sorts of obstacles for the siblings as they try to protect their sister.
It could be Malekith. He might see the sister as a way to get to Thor and Loki and gain power over Asgard. Since she's a new and potentially vulnerable target, he could try to use her in his dark schemes.
Well, one possible plot could be that Thor and Loki's little sister has some special powers that are a mix of their own. She might be caught in the middle of their feuds, trying to bring peace between them. For example, she could use her powers to stop Loki from his mischievous plans that would anger Thor, while also making Thor understand Loki's motives better.
It could be a power - hungry faction leader who sees Four's little sister as a threat to his or her authority. Since Four is already a well - known figure, the sister might inherit some of his influence, and this leader would want to stop her from causing any changes in the established order.
It could be a jealous rival of Jaden who decides to target his little sister to get back at him. Maybe this rival is tired of always losing to Jaden in duels and thinks that by causing trouble for his sister, he can distract Jaden and finally win.
It could be a rogue ninja from their clan who has a vendetta against the Uchiha family in general. Since Sasuke and Itachi are already well - known, the antagonist might see the little sister as an easier target to get revenge on the Uchiha. He could try to kidnap her or harm her in some way.
It could be someone who is manipulating Loki to kidnap the Hulk. Maybe an even more powerful villain from the Marvel universe, like Thanos. He might be using Loki as a pawn to get the Hulk for his own evil purposes.
A mischievous fairy who lives in their backyard could be an antagonist. She might be jealous of the attention the kids get and start causing trouble for the little sister. Like making her toys disappear or creating illusions to scare her. This would force the little sister to be brave and find ways to outwit the fairy with the help of her brothers (even if they don't initially believe her about the fairy).
It could be a new neighbor who is jealous of the little sister's relationship with her brothers. This neighbor might try to drive a wedge between them by spreading rumors or causing trouble. For instance, they could tell the brothers that the little sister doesn't really care about them and is just using them for her own gain.
It could be another powerful deity from either the Norse or the Christian pantheon who doesn't want Gabriel and Loki to team up. Maybe a more rigid and traditional angel who thinks Gabriel is consorting with the wrong crowd by interacting with Loki.
Morgana could be a great antagonist. She might see Merlin's sister as a threat to her own plans for Camelot. Maybe she discovers that Merlin's sister has a power that could undermine her magic or her influence over the court.
Perhaps a group of inter - dimensional bounty hunters. They've heard about Loki's children and the potential power they might possess when they grow up. So they target the children to either sell them to the highest bidder or use their powers for their own evil purposes. Tony then has to fight against this group to save the kids.