Another theme is redemption. Red X is often a character who has room for growth and change. So, many fanfictions involve him trying to make up for his past wrongs and earn the respect of Starfire and Robin.
Well, in some versions, Robin might use his combat skills and strategic thinking. He could have studied Red X's patterns beforehand and used that knowledge to outwit him. Maybe he sets a trap or uses some of his special gadgets to distract Red X long enough to free Starfire.
Perhaps a story where they are both on a mission to a new planet. On this planet, there are unique challenges like a strange energy field that affects their powers. Starfire and Robin Red have to rely on each other more than ever. They start to see each other in a new light and their relationship evolves from just being teammates to something more romantic. As they explore the planet, they also learn about a new culture that has some interesting views on relationships.
Another one is 'Starfire's Secret Mission with Robin and Red X'. Here, Starfire drags Robin and Red X into a secret mission she's been given. It has a lot of comical moments as the three characters with their different personalities try to work together towards the mission goal.
The theme of self - discovery is also prevalent. As they fall in love, both Starfire and Robin learn more about themselves. They grow as individuals while also growing closer as a couple, which is a really engaging aspect in a lot of Starfire x Robin fanfiction.
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.
Romance can also be a theme, although it can be a bit more complex with three characters. Some fanfictions might explore a love triangle between Starfire, Speedy and Robin. There are also stories that focus on their growth as characters. For instance, how Speedy overcomes his insecurities, or how Starfire adapts to life on Earth better, all while in the company of Robin and with Speedy's support.
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.
To write a good character interaction, first understand their personalities. Starfire is often kind - hearted and a bit naive in some ways, while Robin Red is more focused and serious. So, you could have scenes where Starfire's light - heartedness breaks through Robin Red's seriousness, like when she makes a joke during a tense mission and he can't help but crack a smile. Also, show their respect for each other's skills. For example, Robin Red could praise Starfire's combat abilities during a fight.
Robin may also face internal challenges. For instance, if he's doubting his own abilities or if he's emotionally affected by seeing Starfire in danger. This could cloud his judgment and slow down his reaction time. And if Red X has some sort of new weapon or power that Robin is not aware of, that would be another significant challenge in his mission to save Starfire.