Well, it could start with Red X having some ulterior motive like using Starfire to get to the Teen Titans. Maybe he plans to ransom her or use her powers for his own gain. He might keep her in a hidden lair and Starfire would be trying to escape while also trying to figure out what Red X really wants.
Obviously, Red X would be the main antagonist as he is the one doing the kidnapping. But there could also be other antagonists in the background. For example, if Red X was working for someone else, that person could also be an antagonist. Maybe a powerful intergalactic criminal who wants Starfire's powers for his evil plans.
A lot of fanfics might focus on a mission or adventure that they go on together. This gives the opportunity for them to bond and develop feelings for each other in the face of danger.
Other minor characters from the Teen Titans universe might also make an appearance. Maybe some of the villains that are associated with Red X could be there in the background, influencing his actions towards Starfire. Or some of the civilians that the Teen Titans protect, who could be caught in the crossfire of this struggle between Starfire and Red X, and their presence could add more complexity to the story.
One characteristic is the exploration of their opposing personalities. Red X is more of a anti - hero, while Starfire is a heroic and positive character. Fanfictions may focus on how they clash or find common ground. For example, they could be forced to work together on a mission, and during that time, they start to see each other in a new light. Maybe Red X shows a softer side that Starfire didn't expect, or Starfire's optimism starts to rub off on Red X. Also, the fanfictions might delve into the power dynamics between them, considering their different abilities and how they complement or conflict with each other during battles or other situations.
You can try searching on fanfiction platforms like FanFiction.net or Archive of Our Own. Just use the keywords'red x kidnaps starfire' in their search bars. There might be some great stories out there that match this plot idea.
One common theme is trust. Since Red X is a rather mysterious character, Starfire often has to learn to trust him or not. For example, in many fanfics, they start off as distrustful of each other but gradually build trust as they face challenges together.
You can try searching on fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfics from various fandoms, and you might be able to find some great Starfire x Red fanfics there by using the search function.
One great Red X and Starfire fanfic could be a story where Red X is initially a villain but slowly starts to have a change of heart due to his interactions with Starfire. Their relationship could be filled with tension and mystery as they try to understand each other's motives.