One common plot twist could be that she is tied up as part of a ruse by her own side. Maybe they are trying to deceive the enemy into thinking she's been captured. Another could be that she discovers a hidden power or knowledge while in this vulnerable state. For example, she might figure out a way to break free using some newfound skill she didn't know she had before. And sometimes, the person who tied her up turns out to be an old acquaintance with a complex backstory and motives that are not initially clear.
Often, in fanfiction, a plot twist when Black Widow is tied up could be related to a double - agent situation. The person who tied her up might be working for two opposing sides, and their actions with her are part of a much bigger power play. Another twist might be that she's tied up in a place that has some sort of magical or technological properties that she has to use to her advantage to escape. For instance, the ropes might be made of a special material that reacts to her movements in a certain way, and she has to figure out how to use that to break free. There could also be a plot twist where the act of being tied up causes her to have a flashback that reveals a crucial piece of information about her past that helps her in the present situation.
I'm not sure specifically which 'Black Widow tied up' story on DeviantArt you're referring to. There could be various interpretations. It might be a story where Black Widow is in a hostage - like situation and has to use her wits and skills to escape. Or it could be part of a more complex superhero - themed adventure where she's captured by an enemy.
One possible storyline could be about Black Widow's escape from the tied - up situation. She might use her combat skills and intelligence to outwit her captors. For example, she could trick them into making a mistake and then break free.
One common plot twist could be that Black Widow has a hidden motive for the kidnapping that is not initially obvious. Maybe she's trying to protect the kidnapped person from a greater threat. Another twist could be that the kidnapped person turns out to have special abilities or knowledge that Black Widow needs for a mission she can't disclose yet. Or perhaps the kidnapped person is a decoy, and Black Widow is actually after someone else who will come to rescue them.
Another aspect could be that while Gaston is tied up, the villagers who once supported him start to question his actions. Belle might use this opportunity to expose Gaston's true nature to the village. This could lead to a shift in the power dynamics within the village and Belle becoming a more respected figure, not just as a beauty but as a person of integrity.
A great plot twist could be that Black Widow is secretly working for an enemy all along, but she has her own reasons. Maybe she's trying to protect someone or she's been blackmailed. This would completely change the dynamic between her and Captain America.
One plot possibility could be a rescue mission. Maybe other superheroes find out she's tied up and they have to battle the villains who captured her. They would need to use their unique powers to overcome the obstacles and free her.
Black Widow usually escapes by taking advantage of her environment. If she's tied up in a room, she'll look for any sharp edges or objects that can cut the ropes. Sometimes, she'll play mind games with her captors. She might pretend to be weaker or more afraid than she really is, and when they let their guard down, she makes her move. Also, she has a great deal of knowledge about different types of restraints and how to break them. So, she can quickly assess the situation and find the best way to get free, whether it's by untying knots or breaking the bindings.
Perhaps a new, original villain created by the DeviantArt author. This villain might have some special powers or motives related to Black Widow specifically. For example, if the story is set in an alternate universe, the antagonist could be a powerful magic - user who ties her up using enchanted ropes to keep her from disrupting their magical rituals.
One possible plot twist could be that they kiss as part of a mission. Maybe they need to pose as a couple to infiltrate an enemy base. But during the kiss, they start to develop real feelings for each other, which complicates things as they both have their own personal demons and loyalties to deal with.
Maybe Captain Marvel loses her powers temporarily due to some sort of alien technology. Black Widow then has to protect her and find a way to get her powers back. But it turns out that the one behind the power loss is someone they both trusted, like a fellow Avenger who has been mind - controlled.