Another theme is competition. Given their speed - based abilities, it's not uncommon for fanfics to pit Rainbow Dash and Sonic against each other in races or challenges. However, these competitions usually end up with them learning from each other and becoming better friends in the process. It's a way to explore their personalities and how they deal with winning and losing.
Friendship is also a major theme. Their relationship is explored in different ways. Sometimes it's about how they support each other during tough times. Like in some fanfics where Scootaloo gets bullied for her lack of flying ability, Rainbow Dash is there to cheer her up and encourage her.
One common theme is failure. For example, Rainbow Dash might fail at a big race or competition, leading to her feeling sad and disappointed in herself.
One common theme is competition. Since both are excellent fliers, many fanfics revolve around their rivalry in flying competitions. For example, they might compete for the title of the fastest flyer in Equestria. Another theme is friendship. As they interact more, they often learn to respect and become friends with each other.
Often, the theme of perseverance is present. Rainbow Dash, being a competitive character, will strive to succeed in human activities, whether it's in sports or academics. The theme of identity crisis can also be explored. She may struggle with being different from regular humans because of her pony - turned - human nature. And of course, the theme of loyalty remains, but it may be expressed in different ways, like loyalty to new human friends or a cause in the human world.
A common theme is adventure. Since both Sonic and Rainbow Dash are known for their adventurous spirits, many fanfictions involve them going on quests or exploring new places together.
One common theme is friendship. Given that they are both characters from the MLP universe which highly values friendship, many fanfics explore how their friendship develops. Another theme is competition, as both are pegasi and might compete in flying races or other aerial challenges in the fanfics.
One common theme is friendship. When Rainbow Dash is sick, her friends in Ponyville like Twilight Sparkle, Pinkie Pie, etc., often come to her aid. They might bring her soup, keep her company, or use their unique talents to make her feel better. Another theme could be determination. Rainbow Dash herself might show determination to get better quickly, perhaps because she has an important event coming up like a Wonderbolt show.
A sense of discovery is also prevalent. The reader discovers new aspects of Equestria, new magical elements, and also discovers more about themselves and their relationship with Rainbow Dash. This can include learning about new flying techniques if the reader is also a pegasus in the story, or just exploring new parts of the land together.
Romance is also a very popular theme. Fans like to imagine a relationship between them, whether it's a budding romance or a more established one. There could be stories about how they first fell in love, or how they deal with relationship problems. Another theme could be friendship. Their friendship is explored in depth, like how they support each other during tough times, or how they have fun together. They might compete in friendly flying competitions and strengthen their bond through that.
One common theme is friendship development. Since they have different personalities, seeing how they become closer is often explored. Another theme is mutual support. For example, Rainbow Dash might encourage Fluttershy to be more brave, and Fluttershy could help Rainbow Dash be more patient.